ADD LINK Not working

The Link function has stopped working.

If I want to post a URL outside of WordPress, for example:
https://www.markfrancombe.com/oak/KOTH.html

And have the link text be:
ALPHA TEST URL

Then I use:

ALPHA TEST URL:

Instead of direction me to ths URL, I get to:
https://www.markfrancombe.com/wordpress/koth-an-new-game/http/:www.markfrancombe.com/oak/KOTH.html

accompanied with the rather annoying “well this is somewhat embarasing” title..

Why is my domain turning up twice in the link?

Mark