Hi Stephanie -
I want to make sure I understand your question. Are you saying that when the webinar registration form is filled out on your webinar registration landing page, upon submission, the user is directed to the wrong Thank You LP? If so, assuming your webinar registration form is on a Marketo Landing Page - and that the thank you LP is also a Marketo LP, edit the Landing Page, then edit the form on the LP - click the form so that a gear icon appears next to and select "edit" - then check "Landing Page" for Follow-Up Type, and enter the Name (not the URL) of the Thank You LP in the Follow-Up Page section.
If the Thank You LP is NOT a Marketo LP, then choose External URL for Follow-Up Type: and put the URL for the Thank You LP in the Follow-Up Page.
Also, make sure the Form Settings for Follow-Up are set to "Stay on page"
If the Marketo Form is not in a Marketo LP, then use the Form Settings to specify the Thank You Page.