<?php if( @$_POST['regkey'] == 'REGKEY_AS_YOU_LIKE' ) { $auto_belong_groups = array( 4 ) ; } else { $auto_belong_groups = array( 2 ) ; } ?>
Regkey: <input type="text" name="regkey" id="regkey" size="25" maxlength="25" value="<{$regkey|escape}>" /> - set blank if you don't know it.