You can always just copy all of the info in the address (www.http://.....etc.) and paste it in your post as well. Makes for a huge link though sometimes.
Apple devices usually have a little arrow on the youtube video screen that you can touch that opens a menu with an option that says "copy link". Touch the words "copy link" and then later in your post you touch the screen where you would like to paste the link and the word "paste" will come up. Touch that word and it pastes what it copied when you were on youtube.