Brain, I had the same problem of not knowing how to complete the video link on this editor, when I first started to post here.
It took me awhile, but I finally figured it out, but I finally did.
First of all, on the YouTube video itself, right click and a popup menu shows up.
Choose the top option "Get video URL" and click on it.
Another small popup will have that video URL already highlighted.
Right click again and copy that video URL.
Then come back to this Editor drop down a couple lines after the last thing you typed.
Then look on the menu bar, click on the icon that is a piece of filmstrip.
That popup will have a place to paste that copied video URL.
Make sure the cursor is blinking in the address bar of that popup.
Then right click and paste the video URL in there, and click OK.
That is it, your YouTube video will show up linked to your post when you upload it.