You need to type in map coordinates.
On the map maker, you'll see a set of blanks labeled "X", "Y", "Z", and "Yaw". When you hover over a certain spot in the map, that little brown box at the bottom right corner of the screen displays these coordinates. All you have to do is type those numbers as they're labeled in the corresponding blanks where you want the portal to appear and it'll work.
I hope I was clear enough, I tend to be a bad explainer.
