Hi,

To override the _onClickPay function in the checkout_form.js file of the payment module, you can create a custom module in Odoo and define your custom JavaScript code to replace or extend the existing function. Here's an example:
 odoo.define('custom_payment_module.checkout_form', function (require) {    'use strict';
    var CheckoutForm = require('payment.checkout_form');

    CheckoutForm.include({
        _onClickPay: function (ev) {
            // Add your custom code here before redirection
            // Call the original _onClickPay function
            this._super.apply(this, arguments);
            // Add your custom code here after redirection
        },
    });
    return CheckoutForm;
});

Hope it helps