Todo this you need access to the xml layout file for the apps login page.
This will be located in: project -> app -> src -> main -> res -> layout
Within that file you can get the following information needed.
Username resource name: Name of your username (email) EditText resource, skipping the R.id part, so for R.id.edit_text_email input edit_text_email
Password resource name: Same as above, but for password. Again, skip the R.id, e.g. edit_text_password
Sign-in button resource name: Same as above, but for the login Button. Again, skip the R.id, e.g. btn_login
Username: username or email you'd use for testing, e.g. test@mywebsite.com
Password: password you'd use for testing, e.g. testTEST123
This information will have to be entered in the apps Pre-launch report settings: Google Play Console -> All applications -> <app> -> Release management -> Pre-launch report -> Settings
Addition to this, here are some helpful Google links:
Referenced from Voy's answer here.