Thank you page (Custom DOM event) help

Hi, i’m trying to add a simple thank you page to my clients form (her idea), but this seems incredibly complicated for some unknown reason.

Read this page https://contactform7.com/redirecting-to-another-url-after-submissions/
and here it says, add this code to “your theme’s template file”, now something more vague than that would be hard to beat.

There are like 5-10 template files in any given theme, which one do i add it to ?
Do i add it to the page template ? if so, where exactly ? what if my form is in a sidebar ? also we have a few websites, with multiple massive forms and other pages clients are sent to after filling out a form (it’s a process), how do i add a specific page to a specific form ?
None of this is in the documentation.

I used the “on_sent_ok” hook until now, but apparently that will no longer be a thing by the end of this year, so 3 of our websites will become basically useless.

The site is ‘https://tidyup.ca’ if you must know.