You just use the remote community directly: !offgrid@slrpnk.net
Opening that while on waveform.social (and possibly refreshing a few times if you get errors while it initializes it) will show you that community, subscribe to it and generally interact with it.
You can also copy paste the URL from slrpnk.net on the search of waveform.social and it should also show up there, but it never worked that well for me.