UPDATE:
Apparently, there was some dust still setting when most of this thread was written. The big difference is that as of the middle of December 2021, I see the form at MyBinder.org produces links with labpath= as part of the URL when you specify a particular notebook with which to start the launched session.
This specific post above in this thread has been updated reflect the two current options to edit the URLS generated by MyBinder.org to switch the URLs to directing to the classic notebook interface instead of JupyterLab.
I’m noting this here because given the topic title of this thread, others may continue to end up here when looking into how to specify launching with the classic notebook interface.