Cape of Worlds is one of the official maps, the one where the portals to custom maps are stored in.
When you enter mapmaker, in the first tab there is a little checkbox that has the text 'Has Portal' next to it. Click the checkbox. Then put your cursor on the place where you want to arrive when you enter your map from the cape. In the small brown box at the bottom are some numbers now, those are coordinates of the place where your cursor is. Enter them in the boxes that appeared whe you clicked the checkbox, then save.
When you enter the cape afterwards, there should be a new portal you can enter your map through.