This is what the quick reply box looks like:

 

 

You need to click on the icon (indicated by the black arrow) and this will open up the FULL reply box:

 

 

Now click on the little film strip / cell icon (as indicated by the arrow) and this box will pop up:

 

 

Get yaself to Youtube and find the video of your choice... Say hello to King Kurt:

 

 

You need to grab the address (the whole lot) from the address bar:

 

 

And then paste it into the box in your forum pop up window (as shown below):

 

 

Then click on OK and the post box will look like this:

 

 

Then just click on post and you is done!!

 

Good luck, any probs... Please post in here.