Passing form ID to a lookup's "Add New Entry" form

Passing form ID to a lookup's "Add New Entry" form

  1. I have a "Project" that has been created from a form
  2. On edit of that project, a lookup field becomes visible
  3. In the lookup field, I want to add a new entry
  4. In the form that loads in the popup, I want to pass the (source) Project ID to a hidden field
  5. With the Project ID, I can then populate other details on the new form On Add
I can't seem to figure a way to get the ID of the form the lookup is adding the new entry from. 

Any suggestions?