Which version of MyBB you using ? You have to do it by forum. So go to ACP - > Forums & Posts
Find the forum you want to use HTML in then click options to edit the forum. Scroll down the page, Under Miscellaneous Options, the first option is to enable HTML in posts.
Find the forum you want to use HTML in then click options to edit the forum. Scroll down the page, Under Miscellaneous Options, the first option is to enable HTML in posts.
What goes around comes around