the code of view.xml file is:

<odoo>
    <data>
        <record id="project_form" model="ir.ui.view">
          <field name="name">res.partner.form</field>
          <field name="model">res.partner</field>
          <field name="type">form</field>
          <field name="inherit_id" ref="base.view_partner_form"/>
          <field name="arch" type="xml">
              <field name="website" position="before">
              <field name="pan_number"/>
                  <div class="oe_right oe_button_box">
                      <a href="https:/ird.gov.np/PanSearch" target="_blank" class="applicant_docs_forms"  id="P014">Verification Link</a>
                  </div><br/>

                  <field name="vat_number"/>
                  <field name="bank_name"/>
                  <field name="acc_number"/>
              </field>

              <field name="phone" position="before">
                    <field name="key_person"/>
                    <field name="mobile_no"/>
              </field>


              <xpath expr="//page[2]" position="before">
                  <page string="Contract">
                    <group name="top">
                      <group>
                        <field name="contrat_start"/>
                        <field name="contract_status"/>
                      </group>

                    </group>
                  </page>
              </xpath>
          </field>
        </record>





        <record id="sale.order_form" model="ir.ui.view">
            <field name="name">sale.order.form</field>
            <field name="model">sale.order</field>
            <field name="type">form</field>
            <field name="inherit_id" ref="sale.view_order_form"/>
            <field name="arch" type="xml">
                <xpath expr="/form/*" position="before">
                    <header>
                        <button name="open_second_class" type="object" string="Submit" />
                    </header>
                </xpath>
            </field>
        </record>

        <record id="loadingslip_form_12" model="ir.ui.view">
            <field name="name">loadingslip.form.12</field>
            <field name="model">loadingslip</field>
            <field name="type">form</field>
            <field name="arch" type="xml">
                <form string="Mymodule" duplicate="false">
                    <group>
                        <field name="state" invisible="1"/>
                        <field name="id_so"/>
                        <field name="name"/>
                        <field name="partner_id" />
                        <field name="date_order" />
                        <field name="validity_date" />
                    </group>
                </form>
            </field>
        </record>


        <record id="loadingslip_tree_12" model="ir.ui.view">
            <field name="name">loadingslip.tree.12</field>
            <field name="model">loadingslip</field>
            <field name="type">form</field>
            <field name="arch" type="xml">
                <tree string="Mymodule" export="false">
                    <field name="state" invisible="1"/>
                    <field name="name"/>
                    <field name="partner_id" />
                    <field name="date_order" />
                    <field name="validity_date" />
                </tree>
            </field>
        </record>


        <record id="action_mymodule_loadingslip" model="ir.actions.act_window">
                <field name="name">Insert</field>
                <field name="res_model">loadingslip</field>
                <field name="view_type">form</field>
                <field name="view_mode">tree,form</field>
                <field name="view_id" ref="loadingslip_form_12" />
                <field name="target">new</field>
        </record>

        <act_window id="action_loadingslip" name ="loading_slip"
                    res_model="loadingslip" view_mode="tree,form"/>
            <menuitem id ="menu_loadingslip"  parent="sale.menu_sale_invoicing"
                      sequence="20" action="action_loadingslip"/>

    </data>
</odoo>

the code of orm file is:
# -*- coding: utf-8 -*-

from odoo import models, fields, api

class sale_customer(models.Model):
    _inherit='res.partner'
    pan_number=fields.Char(string='PAN Number')
    vat_number = fields.Char(string='VAT Number')
    bank_name = fields.Char(string='Bank Name')
    acc_number = fields.Char(string='Account Number')
    contrat_start=fields.Date(string='Contract start Date')
    contract_status=fields.Selection(
    [('', ''), ('yes', 'Yes'),('no', 'No')],
    default='', string='Contract Status')
    key_person=fields.Char(string='Key Person')
    mobile_no=fields.Char(string='Mobile Number')

class sale_code(models.Model):
    _inherit='sale.order'
    add=fields.Char(string='Address')

    @api.multi
    def open_second_class(self):
        ac = self.env['ir.model.data'].xmlid_to_res_id('sales_ext_agni.loadingslip_form_12', raise_if_not_found=True)
        tbl1 = False
        for o in self:
            tbl1 = o.id
        result = {
            'name': '2nd class',
            'view_type': 'form',
            'res_model': 'loadingslip',
            'view_id': ac,
            'context': {'default_id_tbl1': tbl1},
            'type': 'ir.actions.act_window',
            'view_mode': 'form'
        }
        return result


class loadingslip(models.Model):
    _name='loadingslip'
    _description='loading information'
    _inherits={'sale.order':'id_so'}
    id_so=fields.Many2one('sale.order')