[SalesForce] HTTP Post with redirect(302 Response Code auto ridirect to external URL)

I am accessing an Exteranl API, This Api returns the response code as "302" with HTML Response, This what iam expecting to achive, When i got the response as HTML it should redirect to the Response Page.

Below is my apex class

public with sharing class CreateSalesOrder {
private Boolean Failed = false;
private Integer responseCode;  
private String leadId;
Private String AtlasUserToken;
Private String locationHeader; 
public string ResponseLocation;
//Private HttpResponse responseBody;

public CreateSalesOrder(ApexPages.StandardController controller) {
    Http h = new Http();
    String url ='https://abc.com/salesforce/api';
    //String  ids='00Qm0000009MQpnEAG';        
    this.leadId =  ApexPages.currentPage().getParameters().get('ID');
    HttpRequest  req = buildWebServiceRequest(url,  this.leadId);
    HttpResponse  res = invokeWebService(h,req);
    System.Debug('RESPONSE: ' + res.getBody().trim());   
    System.debug('HTTP Response Code: ' + res.getStatusCode());
    System.debug('Response Body: ' + res.getBody());   
    System.debug('Response location: ' + res.getHeader('Location'));   
    responseCode=res.getStatusCode();        
    ResponseLocation= res.getHeader('Location');
    // responseBody.getBody(); 
    //responseCode == 303;
    if (responseCode == 302){  
          this.Failed =true; 
    }else{
         this.Failed =false;            
    } 
}    

public HttpRequest buildWebServiceRequest(String url, String ids) {
    //Build HTTP Request object
    JSONGenerator gen = JSON.createGenerator(true);    
    gen.writeStartObject();   
    //UserInfo.getUserId();        
    User u = [select Atlas_User_Token__c from user where id=:userinfo.getuserid()];
    AtlasUserToken = u.Atlas_User_Token__c; 

    String jsonS;
    List <Lead> Leads =  [SELECT Company,FirstName,LastName, Street, City, State, PostalCode, Phone, Email FROM Lead WHERE id=: ApexPages.currentPage().getParameters().get('id')];
    for(Lead c:Leads){
        jsonS = JSON.serialize(
            new Map<String, String> {
                'companyName' => c.Company,
                    'firstName' => c.FirstName,
                    'lastName' => c.LastName,
                    'key' => 'fsdfdsfdsfsekZCbFJ3SU9EbUxLZHfdsdfdsfdsfds'
                    });


    }       
    //  gen.writeEndObject();        
    //String jsonS = gen.getAsString();

    // Sending the http body with JSON 
    HttpRequest req = new HttpRequest();
    req.setEndpoint(url);
    req.setMethod('POST');
    req.setHeader('Content-Type', 'application/json;charset=UTF-8');
    req.setHeader('Accept', 'text/html');
    req.setbody(jsonS); 
    System.debug(req);
    return req;
}    

public HttpResponse invokeWebService(Http h, HttpRequest req) {
    //Invoke Web Service
    HttpResponse res = h.send(req); 
    return res;        
}

public PageReference getRedir() {  
    HttpRequest  req;
    HttpResponse  res;
    if (!this.Failed) {
         ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR, 'Sorry.  We got '+ responseCode + ' error Response from atlas. Please Contact System Administrator or try again later.'));
        return null;
    } else {       
        ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR, 'Sorry.  We got '+ responseCode + ' error Response from atlas. Please Contact System Administrator or try again later.'));
        PageReference newPage = new PageReference(ResponseLocation);
        newPage.setRedirect(true);
        return newPage;             
    }

}

private final ApexPages.StandardController controller;
}

This api "https://abc.com/salesforce/api" behave like this, If we post the request Json to that API,it will authenticate key and redirect to https://abc.com(User authentication happen and logged-in to that page ).

In my above code, Redirect HTTP response code and then grabs the location from the headers but its redirection to sign in page.