We have a webform on our site that is not a Marketo form. What would be the best way to capture a person submitted the form in Marketo? Is it possible to create a blind form that would be submitted when the form on our site is submitted? Or is there a better way? We don't need the values that were entered. We simply just need to know if the form was submitted in Marketo so we can add them to a campaign.
If you truly don't care what the person submitted, then you can log a synthetic Munchkin click (using the Munchkin API).
But I think you're forgetting that Marketo won't know who the person is unless you also include their email address. So, as you suggest, you should perform a hidden form post using the Forms API, including at the very least the Email field. If you search the Community there are several examples.
Also please move the post to Products. "Community" refers to the Nation website. Peer-to-peer support is in Products.