I realize the conversion rate could be lower, but why not put this CTA in a non-pop-up form at the end of the article? If I really enjoyed it, that seems like the point where I'd be on board with signing up.
On a web page, I tend to scroll using a single finger or knuckle on the arrow keys. Requiring a mouse at that point (or extra keypresses, if those even work) breaks the reading experience.
If this is a castle, it's not a very inviting one.