Demandbase support provided the answer:
How to enable Demandbase Forms on Modals
Updated:March 31, 2016 00:43
Sometimes you have modal forms or dynamically generated forms with Javascript and Demandbase has no way of knowing when the current form page has a dynamic form of this type. In order to load Demandbase on these forms you can use the manual trigger of Demandbase Forms.
Trigger Demandbase on your modal's callback or when it's rendered.
Demandbase.utils.loadFormModule()
Notes:
- Demandbase field mapping (HTML field IDs) of these forms must match any other current setup we have for the master form template
- You must use this after the form renders otherwise it won't hook
- If you need to hook back into a form on the same page (2 forms, 1 modal and 1 on page) then you'll need to initialize again on the on-page form because it's one library you're using for both. Demandbase doesn't support 2 forms on the same page but you can build your own functionality to do so.