I think it will be better to move this to the namespace holding system messages. You could e.g. have a message called "MediaWiki:Bootstrapskincontact" which holds the link to the page containing the Thank you message. Thus the extension could automatically direct a usere to this given page.
A similar thing is done for core e.g. with "MediaWiki:Disclaimerpage". The page enterted there is the spot to find the disclamer, whereever this may be on the wiki.