To post a link in a message, enter the URL, including the leading https://. This will create a clickable link, for example:

https://www.cnn.com

Entering the URL without the https:// will not produce a clickable link.

Clickable text can be entered as follows:

[Go to CNN|https://www.cnn.com|Click here]

produces this (notice the "Click here" popup tooltip on the link):

Go to CNN