In your campaign, first add a new page:
Add a "Rich Content" element to display your text/image/video to your respondents who have completed your survey.
Then click on "Validate" and don't forget to check the box "Hide the submission button".
To direct your respondents to this custom thank you page, simply add a "Submit and go to page" button on the page before the thank you page to redirect your respondents to it.
Validate, then hide the default submit button built into your survey.
The submit button is present, by default, at the bottom of each page. It automatically returns your respondent to the default end page. The default page includes a predefined message (which you can modify in the general Settings) that will be displayed to all your visitors if you do not hide the submit button. This page also contains a "Back to Home" button that returns the respondent to the beginning of your survey.