vertical-cooperative/easy_my_coop_es/models/coop.py

from odoo import fields, models


class SubscriptionRequest(models.Model):
    _inherit = 'subscription.request'

    vat = fields.Char(
        string='Tax ID',
        help="""
        The Tax Identification Number. Complete it if the contact is subjected to
        government taxes. Used in some legal statements."
        """
    )
    voluntary_contribution = fields.Monetary(
        string='Voluntary contribution',
        currency_field="company_currency_id",
        help="Voluntary contribution made by the cooperator while buying a share."
    )

    def get_partner_vals(self):
        vals = super(SubscriptionRequest, self).get_partner_vals()
        vals['vat'] = self.vat
        return vals