Hello. I'd like to try to help. I can show you step by step. I should point out that this procedure only seems to work for me when I am using a computer. When I try to do the same thing, using a mobile or tablet, I can never get it to work.
The first thing you should do is to "copy" your YouTube link onto your clipboard - from wherever you have it now, perhaps in an email. Let's say this is your YouTube video URL:
https://www.youtube.com/watch?v=-YiFiq_6BzYWhen you are in the screen of creating a message, you will see a row of labeled buttons above your message. At the far right there is a box labelled, "youtube". You are going to use that button in a moment.
"Paste" your YouTube link into the message so it will look like this:
https://www.youtube.com/watch?v=-YiFiq_6BzYThen highlight the entire URL. On my computer, that will put a blue box/highlight over the entire address. Once you highlight the link, take your hand off the computer mouse and the address should remain highlighted. Please note, you should not "click on" the URL. Only highlight it.
While the address is highlighted, click that "youtube" button that I described...above, on the right.
It will then change your link so that it has [youtube] at the left, your full URL in the center, and then [/youtube] at the right. In other words, it would look somewhat like this simulation: [youtube]http//youtube123456[/youtube]
To see if you were successful, click the Preview button below your message. If it worked, you'll see your YouTube video in the box with a triangular "Play" button in the center.
If you do not see the YouTube video play box, get out of the Preview area and try again.
Here is a Youtube video, with this procedure applied.
I hope this helps.
Cheers,
Donna