What you need to do is put only the part of the address after the watch?v= part between the tags. Don't use the full web address. Also, if there is an ampersand (the & symbol) somewhere after the =, leave that and whatever follows it out too. So, include what is after the =, and before the &, if it exists.
So the following would be correct usage of the code for the above video:
This would be incorrect:
Bear in mind that if in some way this code is misused in ways not covered by our current set of rules, we may need to establish a new rule about it. But we will play that by ear. Also, please don't senselessly spam this topic just to test it out. Leaving this topic open is mainly for any commentary on this decision. If you wish, you can use the spam topic in OtBP, or if you wish for something more serious about it, another topic in OtBP with the [video] tag at the beginning of the title. This could prove to be useful for the quest video topic, among some other things. After all, the intention is mostly its convenience when relevant to any particular discussion.