[SalesForce] Custom OAuth Auth Provider

I have a Rest API Endpoint that I am looking to callout to.

Something like this: https://myrestservice.com/service/logs

When setting up the Named Credential,

I set up the URL to: https://myrestservice.com/service/logs

I select OAuth 2.0 as my provider and I am then prompted to select an authentication provider.

I have not created one and when I go to create one and I am provided a list of pre-defined provider types, none of which are my providers.

The documentation indicates I need to create a custom metadata type: https://help.salesforce.com/articleView?id=sso_provider_plugin_custom.htm&type=5#plugin_build

I create the MetaData Type myProvider and add two custom fields to store the Client Id, Client Secret, and Access Token.

Documentation then indicates I need to create a class that extends Auth.AuthProviderPluginClass. Documentation here: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_class_Auth_AuthProviderPluginClass.htm#apex_class_Auth_AuthProviderPluginClass

In my custom provider, I generate an access token, and I am provided with the following:

• Access Token URL
• Client ID
• Client Secret

What I am struggling with is how this translates to the example provided within the documentation that I have provided below..

Upon setting the customMetaDataTypeApiName private string to my metadata api name, I see the fields appear in the auth provider configuration page.

However, in the flow I am working with, there does not appear to be a need for the initiate method, but it is required. Am I misunderstanding Oauth, does a user need to log in to receive a token back? Or can I just pass the clientid, clientsecret, and grant type and received a response?

I tried getting the token response by an HTTP GET Request, but I am returned with a 403 forbidden error.

15:57:02:002 CALLOUT_REQUEST [5]|System.HttpRequest[Endpoint=https://myendpoint.com/token?grant_type=oauth2&client_id=myClientId12345&client_secret=lknvdas898-89fdn-fdnfdb, Method=GET]

  global class Concur extends Auth.AuthProviderPluginClass {

  // Use this URL for the endpoint that the 
  // authentication provider calls back to for configuration.
  public String redirectUrl; 
  private String key;
  private String secret;

  // Application redirection to the Concur website for 
  // authentication and authorization.
  private String authUrl;  

  // URI to get the new access token from concur using the GET verb.
  private String accessTokenUrl; 

  // Api name for the custom metadata type created for this auth provider.
  private String customMetadataTypeApiName; 

  // Api URL to access the user in Concur
  private String userAPIUrl; 

  // Version of the user api URL to access data from Concur
  private String userAPIVersionUrl; 

  global String getCustomMetadataType() {
      return customMetadataTypeApiName;
  } 

  global PageReference initiate(Map<string,string> 
    authProviderConfiguration, String stateToPropagate) 
    { 
        authUrl = authProviderConfiguration.get('Auth_Url__c'); 
        key = authProviderConfiguration.get('Key__c'); 

        // Here the developer can build up a request of some sort. 
        // Ultimately, they return a URL where we will redirect the user. 
        String url = authUrl + '?client_id='+ key +'&scope=USER,EXPRPT,LIST&redirect_uri='+ redirectUrl + '&state=' + stateToPropagate; 
        return new PageReference(url); 
    } 

    global Auth.AuthProviderTokenResponse handleCallback(Map<string,string> 
    authProviderConfiguration, Auth.AuthProviderCallbackState state ) 
    { 
        // Here, the developer will get the callback with actual protocol. 
        // Their responsibility is to return a new object called 
        // AuthProviderTokenResponse. 
        // This will contain an optional accessToken and refreshToken 
        key = authProviderConfiguration.get('Key__c'); 
        secret = authProviderConfiguration.get('Secret__c'); 
        accessTokenUrl = authProviderConfiguration.get('Access_Token_Url__c'); 

        Map<String,String> queryParams = state.queryParameters; 
        String code = queryParams.get('code'); 
        String sfdcState = queryParams.get('state'); 

        HttpRequest req = new HttpRequest(); 
        String url = accessTokenUrl+'?code=' + code + '&client_id=' + key + 
        '&client_secret=' + secret; 
        req.setEndpoint(url); 
        req.setHeader('Content-Type','application/xml'); 
        req.setMethod('GET'); 

        Http http = new Http(); 
        HTTPResponse res = http.send(req); 
        String responseBody = res.getBody(); 
        String token = getTokenValueFromResponse(responseBody, 'Token', null); 

        return new Auth.AuthProviderTokenResponse('Concur', token, 
        'refreshToken', sfdcState); 
    } 

    global Auth.UserData getUserInfo(Map<string,string> 
    authProviderConfiguration, 
    Auth.AuthProviderTokenResponse response) 
    { 
        //Here the developer is responsible for constructing an 
        //Auth.UserData object 
        String token = response.oauthToken; 
        HttpRequest req = new HttpRequest(); 
        userAPIUrl = authProviderConfiguration.get('API_User_Url__c');
        userAPIVersionUrl = authProviderConfiguration.get
        ('API_User_Version_Url__c'); 
        req.setHeader('Authorization', 'OAuth ' + token); 
        req.setEndpoint(userAPIUrl); 
        req.setHeader('Content-Type','application/xml'); 
        req.setMethod('GET'); 

        Http http = new Http(); 
        HTTPResponse res = http.send(req); 
        String responseBody = res.getBody(); 
        String id = getTokenValueFromResponse(responseBody, 
        'LoginId',userAPIVersionUrl); 
        String fname = getTokenValueFromResponse(responseBody, 
        'FirstName', userAPIVersionUrl); 
        String lname = getTokenValueFromResponse(responseBody, 
        'LastName', userAPIVersionUrl); 
        String flname = fname + ' ' + lname; 
        String uname = getTokenValueFromResponse(responseBody, 
        'EmailAddress', userAPIVersionUrl); 
        String locale = getTokenValueFromResponse(responseBody, 
        'LocaleName', userAPIVersionUrl); 
        Map<String,String> provMap = new Map<String,String>(); 
        provMap.put('what1', 'noidea1'); 
        provMap.put('what2', 'noidea2'); 
        return new Auth.UserData(id, fname, lname, flname, 
        uname, 'what', locale, null, 'Concur', null, provMap); 
    } 

    private String getTokenValueFromResponse(String response, 
    String token, String ns) 
    { 
        Dom.Document docx = new Dom.Document(); 
        docx.load(response); 
        String ret = null; 

        dom.XmlNode xroot = docx.getrootelement() ; 
        if(xroot != null){ ret = xroot.getChildElement(token, ns).getText(); 
        } 
    return ret; 
    } 

}