Hello Rocio.
The problem is that, when you select the employee responsible in the popup search, internally the system call the method EH_ONEMPRES_SELECTED, and in this method the system put the employee responsible selected in the field EMPLOYEE_RESP.
So, you have to copy the method to search help of employee responsible and the method to select the employee responsible, and make the changes so the method put the value in your Z field.
Or maybe make your own method, in the GET_v method of your Z Field.
Best Regards,
Renzo.