1. <record id="view_hr_payslip_form" model="ir.ui.view">
2.             <field name="name">hr.payslip.form</field>
3.             <field name="model">hr.payslip</field>
4.             <field name="arch" type="xml">
5.                 <form string="Payslip">
6.                 <header>
7.                     <button string="Confirm" name="hr_verify_sheet" states="draft" class="oe_highlight"/>
8.                     <button string="Refund" name="refund_sheet" states="confirm,done" type='object' />
9.                     <button string="Set to Draft" name="draft" states="cancel"/>
10.                     <button string="Compute Sheet" name="compute_sheet" type="object" states="draft" class="oe_highlight"/>
11.                     <button string="Cancel Payslip" name="cancel_sheet" states="draft,hr_check,confirm,verify"/>
12.                     <field name="state" widget="statusbar" statusbar_visible="draft,confirm"/>
13.                 </header>
14.                 <sheet>
15.                     <div class="oe_right oe_button_box">
16.                         <button name="%(act_payslip_lines)d" 
17.                             class="oe_stat_button" 
18.                             icon="fa-money" 
19.                             type="action">
20.                             <field name="payslip_count" widget="statinfo" string="Payslip" help="Payslip Computation Details"/>
21.                         </button>
22.                     </div>
23.                     <div class="oe_title">
24.                         <label for="employee_id" class="oe_edit_only"/>
25.                         <h1><field name="employee_id" on_change="onchange_employee_id(date_from, date_to, employee_id, contract_id)"/></h1>
26.                     </div>

The code above is a part of hr.payroll, which is responsible in the on_change function in payslip, whenever the employee_id is changed, other fields will be filled up.

Now, I created a function that i wanted to be insert/add here in this part, <h1><field name="employee_id" on_change="onchange_employee_id(date_from, date_to, employee_id, contract_id)"/></h1>.

so that everytime the employee_id is changed, it will not only fill up the fields