Hi @cdymlr
The best way to do this is to use mod_rewrite in the .htaccess file for your root domain to redirect the specific MyBB requests.
Looking at their existing rules that ship with MyBB, you can simply just change them to have your complete URL in them
i.e.
RewriteRule ^thread-([0-9]+)\.html$ https://forums.root.com/showthread.php?tid=$1 [L,QSA]
However, if you weren't using MyBB friendly URLs, you would need to customize the rules to redirect the PHP controllers instead...
RewriteRule ^showthread.php$ https://forums.root.com/showthread.php [L,QSA]
(you would also need to do this for the other supported URLs such as member.php & forumdisplay.php)
You will likely need to tweak and test these to meet your specific scenario, but that should give you an idea of how to address the issue.