How do I set my thank you page to a different link than the pre-set slugs?

Hello,

I have two questions.

1) I already have a thank you page on my site that I'd like to direct my donors to (cccambodia.org/thank-you) rather than a page specific to the fundraiser. How would I do this?

2) Not knowing how to accomplish the first task, I tried pasting the html for the cccambodia.org/thank-you page into the area for the thank you message on the fundraiser. However, when I open the link, nothing shows up: (cccambodia.org/fundraisers/nuts-challenge/thank-you)

Help would be greatly appreciated!
Duyen