Whilst watching on Youtube left click to highlight the address (http:// etc) in the toolbar at the top of the screen. Right click and copy.
On the DTRFG open forum open the Advanced Editor and click on the Youtube logo on the toolbar. A bar appears at the top of your screen asking if you want to allow scripted windows. Click on the bar and choose to temporarily allow scripted windows. Click on the Youtube logo again and this time a window will open. Right click and paste the address you copied from Youtube into the box in the window and click on ok.
Use the Advanced Editor Show Preview facility to check your post as it will appear on the forum to be sure that the embedded video appears correctly before submitting your post.