Call jquery on dependant field

Hi

How i can call the id of a dependant field who is masked by jquery on portal customization ?

 I want to make this :

 jQuery("#helpdesk_ticket_custom_field_cf_XXXX")
                                            .on("change", function(e){
                                            jQuery("#helpdesk_ticket_custom_field_cf_BBBBB").val('Urgent')   
                                          });

But i cnat call helpdesk_ticket_custom_field_cf_XXXX because there a is a standard script use by the snipet to put “<” ine place of “<”

Thanks for help