Hi,

To add custom field to a existing view, already many documentation is available also there is lot of similar questions in the forum itself.

To add a custom field check out this video, in which it is explained : Add custom field

from odoo import models, fields

class TestInherited(models.Model):
    _inherit = 'model_name' 

    custom_field = fields.Char(string='Custom Field')

Then in the XML,

<!--Inherit the corresponding form view--> 
<record id="view_sale_order_custom" model="ir.ui.view"> 
<field name="name">test.custom.form.inherited</field>
     <field name="model">model_name</field> 
     <field name="inherit_id" ref="external_id_of_form"/> 
     <field name="arch" type="xml"> 
          <xpath expr="//field[@name='field_in_original_form']" position="after"> 
               <field name="custom_field"/>
          </xpath> 
     </field> 
</record>

Thanks