Thank you so much
I did that first when I was working on the site which had separate HTML pages (locatable) in the root folder, hence when I linked the "thank you" page, it was able to find it cos it was created as a separate page in the root folder.
Now that the "thank you" page isn't a separate "html" or "php" page that can be found in the (root) or (/blog/) folders, would this method still work as expected?
This is because at the moment, I'm already creating an entirely new "thank you" (html) page that I would keep in the (/blog) folder or in the root folder (when I move WP back to root folder), so I can link to it from the "formmail.php" file.
But if your direction does work, then I would rather do it than creating an entire new page.