Code 1:

class PosSession(models.Model):
_inherit = 'pos.session'

closing_session_note = fields.Char("Close Session Note")

def _post_cash_details_message(self, state, difference, notes):
"""Over - rided method to fetch notes when session is closed
and update closing note in custom filed in backend."""

self.write({'closing_session_note' : notes})
res = super()._post_cash_details_message(state,difference,notes)
return res

Code 2:

<?xml version="1.0" encoding="utf-8"?>
<odoo>
<record id="pos_session_form_view_inherit" model="ir.ui.view">
<field name="name">pos.session.form.view.inherit</field>
<field name="model">pos.session</field>
<field name="inherit_id" ref="point_of_sale.view_pos_session_form"/>
<field name="arch" type="xml">
<xpath expr="//field[@name='stop_at']" position="after">
<field name="closing_session_note"/>
</xpath>
</field>
</record>
</odoo>