void GenerateCredentials( 
   ref SIGNONHANDLERINFO info
)

internal class ChallengeHandler : ISignOnHandler {
  private void GetStoredUserNameAndPassword(string url, out string user, out string pwd)
  {
    user = "";
    pwd = "";
    if (url == "https://your_portal/portal/")
    {
      //TODO retrieve the relevant credentials
    }
    else if (url == "https://your_portal2/portal/")
    {
      //etc.
    }
  }
  
  public void GenerateCredentials(ref SIGNONHANDLERINFO info)
  {
    string referer = "";
    string token = "";
            
    string user = "";
    string superSecretPwd = "";
    GetStoredUserNameAndPassword(info.agoURL, out user, out superSecretPwd);
            
    //Authenticate
    var uri = new Uri(info.agoURL, UriKind.Absolute);
    var ok = ArcGIS.Core.SystemCore.ArcGISSignOn.Instance.SignInWithCredentials(
       uri, user, superSecretPwd, out referer, out token);
    if (ok) {
      //TODO: for example, if you want to retain the token and referer
      //for your own purposes that code would go here...
    }
  }
}
            
  //Within static void Main(string[] args) or your main program
  //Set your challenge handler...
 ArcGIS.Core.SystemCore.ArcGISSignOn.Instance.SetSignonHandler(new ChallengeHandler());
 
 //Establish connection(s) to your desired feature service datasource(s)
 var my_fs = "https://federated_server/server/rest/services/My_Service/FeatureServer";
 
 //Connect to the FeatureService DB
 //If Authentication is required, your ChallengeHandler will be invoked
 //TODO - handle GeodatabaseNotFoundOrOpenedException if authentication fails
 var uri = new Uri(my_fs, UriKind.Absolute);
 using (var fsdb = new Geodatabase(new ServiceConnectionProperties(uri))) {
  //Assuming succesful authentication...
 }
 
 //If you need to clear the handler...
 ArcGIS.Core.SystemCore.ArcGISSignOn.Instance.SetSignonHandler(null);