First, make sure your input field has an ID set.
Then, go in include/class.ticket.php, under the create function:
function create($var,&$errors,$origin,$autorespond=true,$alertstaff=true) {
global $cfg,$thisclient,$_FILES;
/* Coders never code so fully and joyfully as when they do it for free - Peter Rotich */
$id=0;
$fields=array();
//$fields = array('type'=>'string', 'required'=>1, 'error'=>'Problem type required');
$fields = array('type'=>'string', 'required'=>1, 'error'=>'Nom obligatoire');
$fields = array('type'=>'email', 'required'=>1, 'error'=>'Email valide obligatoire');
$fields = array('type'=>'string', 'required'=>1, 'error'=>'Sujet obligatoire');
$fields = array('type'=>'text', 'required'=>1, 'error'=>'Message obligatoire');
if(strcasecmp($origin,'web')==0) { //Help topic only applicable on web tickets.
$fields = array('type'=>'int', 'required'=>1, 'error'=>'Choisir une rubrique');
}elseif(strcasecmp($origin,'staff')==0){ //tickets created by staff...e.g on callins.
$fields = array('type'=>'int', 'required'=>1, 'error'=>'Choisir une rubrique');
$fields = array('type'=>'int', 'required'=>1, 'error'=>'Dept. obligatoire');
//$fields = array('type'=>'string', 'required'=>1, 'error'=>'Indicate source');
$fields = array('type'=>'date', 'required'=>0, 'error'=>'Date incorrecte - must be MM/DD/YY');
}else { //Incoming emails
$fields = array('type'=>'int', 'required'=>1, 'error'=>'Email inconnu');
}
$fields = array('type'=>'int', 'required'=>0, 'error'=>'Priorité incorrecte');
$fields = array('type'=>'phone', 'required'=>0, 'error'=>'Numéro de téléphone valide obligatoire');
Add yours at the appropriate place, and done !