To embed youtube video onto the thread so it can be played without needing the URL displayed, I do it this way.
1: Go to the video on youtube and while it is playing, copy part of the URL that is showing in the address bar at the top of the page. Method: left click address bar to turn it blue then locate the = mark/s. In the case of this video there is only 1, sometimes there is 1 at each end of the bit you need to copy. On this video you need to copy only the end bit 2HjFLK0g-uc. (place cursor between the = and 2, holding left key down and dragging to the end.) This hilights this piece of the code and then you copy it. If there are 2= signs, only copy between them.
2: On the forum thread, open a reply field and go to the advanced field. Write any intro remarks (or not). Drop the cursor down a line or 2. At the top right of the reply box you will see a youtube symbol. Left click that. A [youtube][youtube] will appear where you placed the cursor with the cursor now between the centre ][. Now paste. On this forum I've noticed that you will see the copied code appear in that position, on others the video start screen immediately appears. To check that it has successfully embedded, go to the preview button at the bottom of the box and it should appear there. After checking all is OK, post the reply.
Just a note about the piece of URL that needs to be copied. I've noticed in cases like this one where there is only 1 = mark and not 1 at each end (there would be additional unneeded code where 2 = are used), often some of the required code doesn't show when dragging the cursor. In this case just drag till it stops, then copy.