Embedded forms don't natively support Pre-Fill. This is well-explored here on the Community, but admittedly it's rather subtly addressed in the docs.
Also, FYI... we don't use the term "connector" to refer to a form on a 3rd-party site, so to be widely understood it's better to say "embedded form." Also note embedded forms don't have to use the app-* domain; you're better off loading embeds from your LP domain in case browsers block *.marketo.com as part of tracking protection. (It's OK to avoid TP in this case because the browser's/plugin's goal is to block Munchkin, and that block will still be in effect.)