How to add custom fields to popup form of Stripe Checkout

There is no way to tweak Stripe Checkout unfortunately to add a custom field or a checkbox. The solution here is to use Custom Checkout and add those extra fields to your own form. You would for example collect the customer's name and ask him to accept your own Terms of Service and only allow them to click on the Pay button once they do.

Then, once the customer fills Checkout with their card details and Stripe sends you back the token you would send it to your server along with the extra details you collected on your end.