I usually use the following line of text in the Source of a Rich Text field: 

<a href="{Common:ContextItemUrl}" target="_blank">{Common:ContextItemDisplayName}</a>‍

As Shown:

What you'll see after you hit OK: 

What it looks like in the editor: 

What it looks like on the Task Form proper: 

For whatever reason the 'Insert Hyperlink' in that dialog has never worked and does nothing when I click it. This was a quick and dirty way around that though. 

Hey N M‌ - it looks like you're in On Prem - not quite the same in O365!
Any reference text I try to add turns into this weird ... mush. 

open your task form, add a rich text control to your task from. I double clicked to get into the properties of the rich text field. you can then add your workflow var to the body of your rich text control

Never mind got it resolved. The URL need to be the full URL not a relative one when you construct and add them to a email or task body. Relative will still work on a formatted task form if you have a text box enabled to be rich text.