Hi

Check the code below,

<template inherit_id="payment.payment_tokens_list" id="payment_tokens_list" name="Payment Tokens list">

    <xpath expr="//button[@id='o_payment_form_pay']" position="inside">

        <!--        Your Code-->

    </xpath>

</template>

The "Pay Now" button is called from a template "payment_tokens_list" in the "payment" module. the button name is dynamic and it is used in many other areas too. so the changes will be affected there too. you can also check the template with id "payment" in the "website_sale" module from where it is called.

Hope it helps