There can be multiple ways to get this done. But I will go with a simpler approach.
I think with some custom javascript logic, you can achieve this.
On your thank you page, include a javascript code which will check for the 'browser history' object and check if the history contains the url of the form you want to be submitted. If that form is not there in history, it means the user directly arrived at your 'video' pages by pasting the url etc. Thus if the history, does not contain the form url, just redirect to the webform. Otherwise, simply display your videos.
There can be ways one can sneak past this, and with some other additions or all together different approaches, that can be enforced more tightly. But I always lean towards simpler and effective solution than complex, 100% foolproof on especially in marketing related workflows.
Let me know if you have any questions. Hope this helps.
Rajesh