You would create a new article and have it as category unassigned (in J1.5) or build it as Static Content (J1.x). Then you publish that page to a menu, preferably a menu that only admins have access to. Bring up the page, and copy the link from the browser. You can then take the page off the menu. Then you go into the Acajoom module, and find the custom thank you page URL slot, and paste that in. CorFun normally does this for you as part of the Website build process.