When you are logged in and viewing a thread/topic/post, you will see a "report" icon on the left side, next to the Quote button
(white turns red when you mouse over it)
If you click that, the report post screen loads. In that form, one field is the reason for reporting the post, which is a selection list, and one of the options in that list is "This is an important message and should be stickied" There is another area for additional information, and you can choose whether or not to be notified when you "report" is dealt with.
So, the official way to request a thread be stickied is to report the post identifying it as a sticky candidate. However,this isn't reliable. A couple of weeks ago I reported The Great How-To modification Index
to be a sticky but it's still out there non-sticky, and if I try to report it again I just get a screen saying its already been reported.
So maybe there's a problem with that method. The For Sale/Wanted Forum Guidelines
ought to be sticky, but instead of reporting it, I bumped it.