how do you add button to the in post ad section?

I either need to add a button to the in post ads visual on my posts, or make the area smaller using CSS.

In you screenshot example, I see there is a "blue button" that says find out more. But I can't see where to turn that on?

Also I assume why there is so much space at the bottom of my ads, because a button is supposed to be there.

Any help would be appreciated.