create a page and call it the same as your forum slug (usually forums)
then in that page put the message you want followed by
(If you are using Gutenberg, then as this as a shortcode block)
this will then be your forums page
My plugin GD Forum Notices Pro allows you to define rich messages to display anywhere you want in the forums, with rules to define how the messages/notices will look like and the rules for display to different user roles. You can also schedule date range to display message.
No coding needed, all done via the admin interface. Check it out here: https://plugins.dev4press.com/gd-forum-notices-for-bbpress/