Yes and no. As far as embeds go, you can’t really embed without linking to the outside source here. On top of that, the forum has a trust level system that dictates what you’re able to include in your posts. At trust level 1, you are able to embed links in your posts to trusted sites such as youtube (as you did above) or links to pages on WoW’s site. More options open up at trust level 3.
The important thing though is that just because you can post them, they do still have to follow the forum CoC.
So where the lines are blurred here in the help forums are if someone is asking for specific information and you know of a video segment that contains that information.
Example; if someone were to ask “What is the history between Jaina Proudmoore and Arthas Menethil?”, it’s often a lot easier to just direct them to a video that covers that topic than it is to try and explain it in a 30 paragraph reply. That’s directing them to specific information about their question, rather than advertising for the video or channel.
However, If I were to create a new unsolicited thread and say “Hey guys, check out this video I found about the history between Jaina Proudmoore and Arthas Menethil.”, and then link to the video, that would 100% be on the “advertising” side of the line.
More information about forum systems and trust levels can be found here:
The forum CoC can also be found here:
Keep in mind it’s intentionally written to be vague so that people can’t try to “lawyer” their way around the rules.
All that said though, should you like to stick around in the future and either ask whatever questions you might have, or help to answer questions that others might have, you’re more than welcome! It’s always good to have more sets of eyes on the problems people come here to ask. And as Nok said, a one off mistake because you didn’t know is unlikely to bring any consequences down on you. 