Here are the instructions to linking in a URL inside a forum topic. 

1. create your message in the forum and paste into your message your link or any phrase. 

2. high light your phrase or URL in your message ie https://youtu.be/pvt-d63Ausc

See the first attached image below. 

3. notice that at the top of the message box that the links are now active and you can select the links and then choose the Insert link menu. 

See the second attached image below. 

Notice that in the Link Text box you see your highlighted phrase. 
In the Website ULR or Email box you add in the url that you need and choose Insert Link. I prefur to copy paste into this box for accuracy.

These instructions work for Windows 10 at least. I can't test on a MAC... 

Let me know if you have issues. 

Jay