Matrix Gateway: Join Matrix Rooms from XMPP

[ch0ccyra1n]

Before creating a ticket, please consider if this should fit the discussion forum better.

Is your feature request related to a problem? Please describe.

I'm not sure if it's possible and undocumented but I couldn't figure out how to join a Matrix room via the Matrix Gateway. Trying something like thecoolestroom%matrix.org@matrix.xmpp.tld for #thecoolestroom:matrix.org yielded no results.

Describe the solution you'd like

It would be great to have it be possible to specify a separate subdomain such as conference-matrix.xmpp.tld or for joining matrix rooms over the gateway.

Describe alternatives you've considered

Alternatively it could be possible to use a symbol other than % to indicate that what the user wants to join is a room rather than a direct message with the user, although I could see this being a bit problematic for those with matrix_id_as_jid set to true.

If it's already possible to join a room through the gateway, I'd appreciate it if this was documented somewhere such as in the blog post and/or documentation.

Additional context

For reference, I have matrix_id_as_jid set to false in my own server.
Otherwise the gateway so-far has been amazing!