Hi there. I have done a quick search to find out how this is achieved and there are a few possibilities but I cannot be 100% yet that it would work.
One possibility is to host the thank you page on my own server and redirect to it. Others are to actually customise the page directly within Google.
As it's a bit of an unknown, I'd need to get into it and do more research too.
In order to invest time into this, what I'd like to do is to ask you to award the project to me (if you choose me of course) and I'd then try to implement it, if I succeed, I'd accept the project and we can close it off, otherwise you may award to an alternative freelancer. That way we're both protected.
===
I worked for 11 years as the sole full-time in-house developer for a yacht charter company based in Cannes, working with PHP/MySQL/HTML/CSS/Javascript both front-end and back-end. I created their customer site complete with search, bookings, content/CMS, accounts/financial tools, sales rep tools with automated proposals, the list is endless. Since 10 years I have been running my own games website for a dice game, which I designed and built, it includes a forum, AJAX chat and AJAX game, with automated daily tournaments. I have been on Freelancer.com since May, please check out my reviews.
Please send me a message with any relevant details, if you like my proposal, even if I am offline, and let me know when is a good time to catch you. I will reply to any messages ASAP.