How can I collect the coupon codes on the V3 checkout pages during signup?

Problem Statement

You want to know how can I collect the coupon codes on the v3 checkout pages during signup.

Solution

To collect coupon codes on the checkout pages turn ON Allow customers to add/remove coupons (screenshot below) under Settings >> Checkout & Self-Serve Portal (under Customer-facing essentials) >> Configuration and Publish it.

Once this is done, this is how your checkout page will appear.