odoo16/addons/payment_custom/views/payment_custom_templates.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

    <template id="redirect_form">
        <form t-att-action="api_url" method="post">
            <input type="hidden" name="reference" t-att-value="reference"/>
        </form>
    </template>

    <template id="custom_transaction_status" inherit_id="payment.transaction_status">
        <xpath expr="//div[@id='o_payment_status_alert']" position="inside">
            <t t-if="tx.provider_id.sudo().code == 'custom'">
                <div t-if="tx.reference">
                    <strong>Communication: </strong><span t-esc="tx.reference"/>
                </div>
                <div t-if="tx.provider_id.sudo().qr_code">
                    <t t-set="qr_code" t-value="tx.company_id.sudo().partner_id.bank_ids[:1].build_qr_code_base64(tx.amount, tx.reference, None, tx.currency_id, tx.partner_id)"/>
                    <div t-if="qr_code" class="mt-2">
                        <h3>Or scan me with your banking app.</h3>
                        <img class="border border-dark rounded" t-att-src="qr_code"/>
                    </div>
                </div>
            </t>
        </xpath>
    </template>

</odoo>