I'm trying to do this on a Conditional Thank You page, in other words,

I'm trying to do this on a Conditional Thank You page, in other words, one where the user is diverted based on a condition on the page, and every time I try to save the Javascript, it disappears. Is this no longer possible?

May I know where are you trying to save the Javascript code? You will need to create a separate html file for the given code in the guide: Create a Custom Thank you page with Auto-redirect and also for the javascript to work. You will then upload the created html file to your own web server and use the URL to the html file to your Thank You page setting.

I misunderstood the instructions. Thank you. This is helpful!