For embedded forms, the coupon box is an option you can add when you are getting the code for the embedded form.
Here's how to create an embedded form for an offer:
From the overview menu of an offer, click the 'Create Button' link. (This can also be reached from the main menu under Create >> Forms, Buttons & Links.)
If there are multiple options showing, select 'Create order button, form, or coupon box'. (If not, this is the default option.)
On the Create >> Forms, Buttons & Links page, there will be an option, 'Show Gift-Certificate/Coupon Box'. Check this option, and your embedded form will include the coupon field.
The resulting code in the box at the bottom of the page can be pasted into your landing/sales page.