For youtube videos:
Now the link part will not be the full youtube URL. Here's an example:
http://www.youtube.com/watch?v=FksFG1l7xKI
This part
FksFG1l7xKI is what you would use after the = in the forum code. If you do it incorrectly, the youtube block will be pure white, indicating that it's not correct. When you do it right, it should look like this:
For images:
You use img in brackets, the direct link to the image (some other links may work, so direct links are your safest bet) then at the end you type img again in brackets, except you use a slash like this /img. Both must be in brackets [ ].
Spoiler tags function in the same way. Spoiler - /spoiler in brackets.
For future reference, if you have any forum coding questions there is a sticky that was made for our forums that explains all of the codes one can use to format their posts to their liking. You can find it here! [http://www.escapistmagazine.com/forums/read/18.198114-Escapist-User-Guide-to-Posting-Commands]
I hope this helps!