Slidge is an XMPP server component that can be used to send and receive messages with
any XMPP client,
to name a few.
Your contacts on the legacy network are given a “puppet JID”
of the type
that you can use to interact with them, just as you would with
any normal XMPP user.
username depends on the slidge plugin you use, for instance
on signal, it is the phone number of the user you want to reach.