Apex – How to Add Dynamic URL in Self Registration Welcome Email for Experience Cloud

1. You're not setting the parameter to send the email in the method createExternalUser and it seems the default is set to false to not send.
2. The Site Class docs show that you should be able to send the email by adding a 4th parameter of true:

createExternalUser(user, accountId, password, sendEmailConfirmation)

From the question, it seems you're trying to send a parameter via URL to a community page and trying to retrieve the same. It can be done in below steps -

1. Create a community page where you want to navigate and put the LWC component to receive the URL parameters
2. Use the community page name in your source component, from which you want to redirect. It can be achieved via below code -

this[NavigationMixin.Navigate]({
    type: 'comm__namedPage',
    attributes: {
        pageName: 'target_page_name', //replace this by page name created in step #1
        recordId: this.recordId
    }, 
    state : stateObj
});

3. At the receiving end, write this below code which will help you to fetch the parameter from URL.

//Generic method to give the URL parameter value
getUrlParamValue(url, key) {
        return new URL(url).searchParams.get(key);
}

//Getting URL parameter here.
//Assume - Parameter Name is "Id"
const idFromURL = this.getUrlParamValue(window.location.href, 'Id');
console.log('ID from URL :' + idFromURL);