1. Override the autocomplete controller in Python:
from odoo.addons.base.controllers.partner_autocomplete import PartnerAutocomplete

from odoo.http import route, request

class PartnerAutocompleteExtended(PartnerAutocomplete):

    @route('/partner_autocomplete', type='json', auth="user")

    def partner_autocomplete(self, **kwargs):

        result = super().partner_autocomplete(**kwargs)

        for partner in result.get('partners', []):

            rec = request.env['res.partner'].sudo().browse(partner['id'])

            partner['x_custom_field'] = rec.x_custom_field

        return result

2. Extend the JS widget to use the custom field:

odoo.define('your_module.PartnerAutocompleteExtended', function (require) {

    "use strict";

    const FieldPartner = require('base.PartnerAutocompletion');

    FieldPartner.include({

        _parseResponseData(partner) {

            const result = this._super(...arguments);

            result.x_custom_field = partner.x_custom_field;

            return result;

        }

    });

});

3. Load your JS in assets:

<template id="assets_backend" inherit_id="web.assets_backend">

    <xpath expr="." position="inside">

        <script type="text/javascript" src="/your_module/static/src/js/custom_partner_autocomplete.js"/>

    </xpath>

</template>

Hello @Odd Dellarobbia,

Just a quick clarification:

The widget partner_autocomplete_many2one is not part of the standard Odoo framework.

Instead, Odoo uses widgets like field_partner_autocomplete or res_partner_many2one for partner autocomplete functionality, including search and selection, especially on website forms.

For Example:

Python Side :-

//Code_1 in Comment//

XML View Inheritance:

//Code_2 in comment//

If you have any questions or need further clarification, feel free to reach out.

Thanks & Regards,

Email:  odoo@aktivsoftware.com           

Skype: kalpeshmaheshwari