On the very end of the toolbar when editing you can see a link icon (well sort of). Highlight the text, click that icon, add URL and the words you have highlighted should appear on the email and will launch the HTML you put in
On the assingment window below for request, i cant seem to find the "icon" button to add the link:
However, on the incident assignment window, i can see the icon button to add the link:
I double checked to see if HTML was enabled and it currently is:
Anything im missing?