It's tougher on a phone, but it not that hard. The principle is the same whether on a PC or phone.
Use your browser and you go to someplace you want (can be anywhere). Look at the top of whatever browser you're using, you'll see a link that starts with
http://www......... Just highlight and copy the whole link. Then go to the posting you want to include it in and simply hit paste in the area where you are typing your response. It's the same as any other text. I usually put links on their own lines so the link doesn't get broken by any automatic text wrapping. Short of that, it really easy once you do it once or twice.
For example, I just went to YouTube and found the vid below. While watching I figured others might also enjoy it so I highlighted the link address at the top of the page while I was watching.
Next I came back here and pasted it in:
https://www.youtube.com/watch?v=pPEDtKvt4lI
And that's it!
Nice yard and cruising vid! (the tractor is looking and running damn fine!)