Its really quite easy

Step 1 get the url from youtube by copying and pasting from either the address bar or the url box at the right, don't copy the embed code.

Step 2 Paste into your post and then delete the bit before the = sign so you are left with

Step 3 you now have 2 choices

3A type the following before and after the youtube reference above

or 3 B go into the go advanced editor (if its a new thread advanced editor is open) then double click/highlight the youtube reference you pasted and click the youtube icon (its the 2nd from right hand end)

this will wrap the code around it so you final link looks like

if its youtube HD you can use the 3rd from right icon, if its youtube hq the final icon

those codes look like

Submit the post and sit back and watch