Hi,

Try the following code,

1. Create python file.
from odoo import fields, models
class SaleOrder(models.Model):
   _inherit = 'sale.order'
   def function_nam(self):
      print('HELLO')

2. create Xml file.

<?xml version="1.0" encoding="UTF-8" ?>
<odoo>
   <record id="action_update_order_date" model="ir.actions.server">
       <field name="name">Print HEllo</field>
       <field name="model_id" ref="model_sale_order"/>
       <field name="binding_model_id" ref="sale.model_sale_order"/>
<field name="binding_view_types">form</field>
       <field name="state">code</field>
<field name="code">model.function_name</field>
   </record>
</odoo>

Please refer to this blog:

https://www.cybrosys.com/blog/how-to-add-a-button-inside-action-menu-in-odoo-17

Hope it helps