public bool IsSignedOn()

//Find the portal to sign in with using its Uri...
var portal = ArcGISPortalManager.Current.GetPortal(new Uri(uri, UriKind.Absolute));
if (!portal.IsSignedOn())
{
  //Calling "SignIn" will trigger the OAuth popup if your credentials are
  //not cached (eg from a previous sign in in the session)
  if (portal.SignIn().success)
  {
    //Set this portal as my active portal
    ArcGISPortalManager.Current.SetActivePortal(portal);
  }
}

var portal = ArcGISPortalManager.Current.GetActivePortal();
//Force login
if (!portal.IsSignedOn())
{
  portal.SignIn();
}
var user = portal.GetSignOnUsername();