The submission URL is the endpoint where you can send errors for Breakpad, Crashpad, or direct API.
For Backtrace-hosted instances, your submission endpoint for Crashpad looks like:
https://yourteam.sp.backtrace.io:6098
And for Breakpad and direct submission to the API or via curl :
https://yourteam.sp.backtrace.io:6098/post?format=minidump&token=
..where yourteam gets replaced by your universe name, and is your submission token.