Check if WebGl Instance has fully loaded

We are making a WebGL Game for Christmas. Once a user has logged in over our website, the unityInstance is loaded. We'd like to send the id of the logged in user to our Game and tried using the UnityInstance.SendMessage() method. But for the JavaScript to call this function we need an event. Is there a way to catch the event of the Instance having loaded fully? Or would we have to call the function ourselves from inside the game after adding the .jslib extension? Many thanks