Tom,
This is how I do it, using an Apple iPhone.
First, find your YouTube video on the web and set it playing on your screen.
While it's playing, bring up your "history " on the screen, tapping on the video you've just been watching: this'll set it going again on your ( new) screen.
Tap the "send to " icon. On the "message to/mail to" screen tap "message to", and send it to yourself.
Shortly, you'll hear your message inbox "ping".
Open your messages, firmly tap the message with the still of the video showing. This will bring up a dialogue box with a "copy " option : tap the "copy " option.
Now, wend your way back to the forum with your message or answer (or start a new message, or an answer )
At the point in your message ( or answer) where you want to insert the YouTube video, firmly tap and hold the cursor and a "paste " option will appear.
Tap "paste ", and "Bob's your uncle" (the video link appears)!
Press " send reply " ( or post) and the link will appear embedded in your message (or post), in blue.
All this takes far less time to do than to describe
