In Google firebase test lab, for custom login of my android application, I am not able to access/get the resource IDs of username and password field. This is happening because my sign in page is web-view. Is there any way to access the resource ids in web-view or any other approach to perform compatibility testing
I'm assuming you're asking about a Robo automated test and not instrumentation tests. Firebase Test Lab does not support WebView at this time. It's noted in the documentation.
If this is a feature you'd like to see, you can effectively vote for it by filing a feature request.