Airwards

This is the first time posting on here so excuse if I have missed anything.
After literally 2 months trying to work this out and having no luck either with SQSP as well as developers both here and personally, one friend of mine and I seem to have solved it.
Create an unlinked page on your website where you want people to go following confirmation of their donation. For example www.yoursite.com/thanksfordonating
We worked out that while you cannot edit or inject any code into the donation-confirmation page itself, the one thing it has in common is a header and footer. 
Therefore into the Settings > Advanced > Code Injection > Header we added the following javascript code
And it seems to work great.