Are there any methods you know of to automatically add someone's email address to a MKTO form?

If the form and the portal share a common parent domain, you can store the value in a cookie on the portal side and then read it in the form page (using JS). This is the most streamlined method as the lead never sees their own address in transit (not that that's necessarily a problem, but it's better to avoid the obviousness of the approach).

It they're under different private domains, like www.example.net and pages.example.info, then pass the email address in the query string (or #hash) of the URL that takes the person between sites. Then follow the same pattern, just set the value using Forms JS (setValues()).

Actually just realized your post is in the wrong place.

I'm not going to remove my response but will flag your post to be moved. In future, make sure to open your threads in Products: https://nation.marketo.com/t5/Products/ct-p/products