Create a coupon on Etsy
Navigate to Marketing > Sales & discounts and select create a promo code:
Create a new coupon code, set your discount amount, duration and promo code:
Select which listings to include, then confirm and create code. You will see a new promotional URL created for your offer, save this link (you will need it later).
Add the coupon to your email campaign
You are free to add the coupon any way you like in the editor, but if you want to automatically apply it to a button you can create a new button element, add the coupon code as the button text and then add the promotional URL as the button link: