The auto pay feature in Xero is enabled once you set up your stripe account and create a repeating invoice that is set to Approve for Sending as seen here:
Once you've entered all the details for your repeating invoice you can hit save which will save the template and email the Agent on the Invoice date selected with the email template you select. Xero has a default template called Auto Pay: Basic. To edit these email templates you can find them under your settings menu.
The Agent will receive an email with a note that they can set up auto pay from the online billing portal.
Once they click on the payment option they have the option to pay now or to set up auto pay.
To set up auto pay they can simply enter their card and address details and check the option to subscribe to auto pay.
Once the payment is made the online invoice shows that autopay has been enabled and they have the option of editing their card information or cancelling autopay by opening the online invoice anytime in the future.
This will create a contact record in Stipe using their email address, and will store the successful transaction. Please note that it does not create a "Subscription" in Stripe.
For more help with Xero and Stripe please refer to this article for support: