Is it possible to add them in the form output?
If you simply want to know where someone used a form just change the subject to a recognisable ID e.g home page form.
Some clients requested this feature. They want to know from where the leads usually come from. LIke, was it organic search or a specific ad, etc. And they want to see this info in a lead sent by a form.
If they are sending leads from specific places and you know the source that wouldn’t be difficult, but if they simply expect the form to collect tracking information from miscellaneous sources I think that will be tough.