Not sure if you found a solution for this but you can try the following (tested and works);

1. Open up Login.aspx and locate the following code:

<input id="LoginButton" style="z-index: 101" type="submit" value="Submit" name="Submit1" runat="server" class="LoginButton">

2. add the following "onclick" attribute and value to it:

<input id="LoginButton" style="z-index: 101" type="submit" value="Submit" name="Submit1" runat="server" class="LoginButton" onclick="appdom();">

3. Locate the ending </head> tag and place the following JavaScript above it (change the @xyz.com value to your domain):

<script>
function appdom() {
  var t = document.getElementById("TheLoginInterface_UserBox");
  var h = t.value += '@xyz.com'
  t.value = h;
  Form1.submit();
}
</script>

4. Save the file and reload the page.

Hope this helps!