Errors when configuring a text field to show a hyperlinked URL

I'm trying to use a calculated text field to display a hyperlinked URL. When I use the below formula it works perfectly.

• concatenate("<a href=","https://en.wikipedia.org/wiki/Horse"," target=""_blank"",>Click Here</a>")

However, when I use this formula, and make the HF equal the URL above exactly, it doesn't work.

• concatenate("<a href=","[HF_Custom URL]"," target=""_blank"",>Click Here</a>")

Any advice?