You mean the drop-down box that lists the servers? In the config/main.inc.php file, just make this variable (or setting) your domain:
$rcmail_config['mail_domain'] = '';
So for example, here it would be:
$rcmail_config['mail_domain'] = 'roundcubeforum.net';
If you wanted multiple hosts, it would be something like:
$rcmail_config['mail_domain'] = array('imap.roundcubeforum.net'=>'Roundcubeforum.net');