If you do not yet have a Jabber account, you can register one. The registration processes for various servers differ, but many servers support so-called “in-band registration”, which is described in this section.
To register an account, type C-u M-x jabber-connect and enter your desired JID in the form ‘username@server’. You will be presented with a registration form to fill out and send. There the username you chose will be prefilled. After registration, you can connect to the server as usual.