For International ticket buyers, first, select the ticket and then click on Proceed. Now, you will see a checkbox "Are you making an International transaction (Card issued by banks other than Indian Banks)?" in the Buyer Details Section. See the screenshot below:
Once done, fill out the form by adding the complete address as per the form. (If you do not see this checkbox, this means, International Gateway is not enabled for the event.)
Now on the payment section, please select the second gateway "Stripe". See the screenshot below:
