Do you mean an all-points bulletin to search for someone? I think only law enforcement can put out an APB.

Or do you mean something like a classified ad, like putting something up on a bulletin board? If so, that's just a matter of making a post here like you did in this case. There are lots of categories and groups to help you target the audience you're trying to reach.

Just make a Post, or write an article if it is long.....

You just did ☺

There is a "magazine" option on the home page where you can submit articles for consideration. That's the only "bulletin" function I know.