Issue with iframe on submission of form


First time posting, so apologies if any of the tags etc are incorrect.

We have created an iFrame for a webinar registration form, which looks good, but on the submission of the form, the whole iframe turns grey and the user is left with the message: refused to connect.

Does anyone have any tips or advice as to why this is happening? I have tried this on chrome and edge, and the same has happened on both browsers.

Here is a link to the page where the iframe sits - Webinar: Introducing our new platform - Builder's Profile