Don't ask. Tell people about the value proposition in ten words or less at the top of each article. Make one of the words a link to a page with more information. On that page have a simple field to sign up for email updates.

What I want is something that prioritizes my needs, not those of the person creating the content.

A popup when I go to leave is not a fair exchange.

Because I didn't agree to it ahead of time.

And now you are just blocking me from what is important to me with a toll booth.

Good luck.