Don’t really understand the question. How would Marketo know that the person watched or not, if you know you’re not sending that data?
I guess that's what I'm trying to figure out. Is there a way to send the information to Marketo? Is it a matter of embedding the link within the "recorded webinar link" token in the campaign? I am just unsure of the process if the webinar is not connected to ON24, GoToWebinar, etc. It seems that all webinar discussions and examples are through those vendors.
You could potentially capture an on demand view by sending a Marketo email as a confirmation (containing a URL for the webinar LP) when your form is filled. You could then count anyone who clicks that URL as an on demand view.