Best way to measure real KPIs? Don't add a link to your product

Whenever I build a new product, I follow this advice:
"Make the simplest version of your product first. Learn and iterate fast."

I am building a course for Founders and I am coding my own platform for it and the simplest way to see if the content is interesting was to start a simple newsletter course.

I got more than 100 founders going through the course but I wanted to see how many of them are really interested about a full course.

So, in the last email where I talked about the course, I DIDN'T ADD THE LINK to the full course.

I am not interested in how many people will click to check out the website. I am interested in how many people want the course so much that they will send me an email.

And it worked. 30 founders send me an email:
"I was trying to access the full course, but it seems like there isn't any working link in this email.
Is this correct?"

"is the last email supposed to have a link to the full course? If so, it might be missing"

So, there you have it. Force your users to really ask about your product
(by the way this trick is also part of the full course - but I won't include a link here ;)