Step1: Every Webinar you create has some tools with it. The second tool with a pencil icon reads 'Edit/view Webinar Details'
Step2: Click on the 'Webinar Registrants' icon indicated in the below image.
Step3: Open the 'Thank You Page (optional)' Dropdown
Step4: Provide the URL of the page where you want your user to be redirected after filling the Registration form.
Step5: Click 'Save Now'