You can place these code snippets directly in your sbnc.tcl. Alternatively you can create a tcl file for it and load it in sbnc.tcl, there are plenty examples. Don't forget to run /sbnc tcl :rehash after making the changes.
internalbind usrcreate set_default_server; proc set_default_server {user} { setbncuser $user server "irc.quakenet.org"; setbncuser $user port 6667; }
internalbind usrcreate set_default_realname; proc set_default_realname {user} { setbncuser $user realname "Your realname" }