I noticed nobody has responded yet, so I'll try to help.

The default domain should be the domain your .entrypoint functions exists
within, as the default domain is generated automatically to host it, I
believe. I guess if you're in your .entrypoint function, it's possible to
make a request for the current domain, and then that should be the default
domain.

I could be wrong as I've never tried this myself. I hope I've helped.