Regarding posting on forum, you just did it.
And regarding custom scenarios, you just go to Stronghold Crusader downloads section (one can see the link from forums home page. Then you pick a category you want, you chooses map/scenario you want to download, and clock the download link - it slightly above the deception of the file.
Then you go to your downloads folder, find the zip archive for you just downloaded, unzip it, and copy the .map (I think that's the extension for those files) file, and copy it to your Documents/Stronghold Crusader/Maps folder.
Next time you run the game you will be able to play the map you just downloaded. If it is a scenario map, there is an option in the game by which you can play a custom scenario.
If you any further assistance feel free to tell.