On the form edit page, click on Share / Embed on the top-left corner, and go to Embed → Popup Form. Here you will find the embed code for popup forms.

Popup forms have a two-part embed code. The first part can be inserted anywhere on the page (but preferably towards the end) and the second part is the link which triggers the form, and should be inserted where you want the link to appear.

If you look at the first part of the embed code, it has a unique embed ID - htwpp. This ID is then used in the popup link, as an attribute - data-fc-open='htwpp'. If you want to trigger the popup form using a custom element on your page, you would use this attribute on that element: data-fc-open='htwpp'.