This only works in Forge mode.

Start Forge, put the map on "Forge World"... you should spawn in the metal cave.

If not, fly up to the metal cave inside of the mountain.

Go to the edge of the opening of the cave, press up on the D-Pad to turn into monitor/forge mode).

Go about 4 feet under the edge of the cage and there should be a big, glass window in front of you.

Select the receiver teleporter, put it all the way inside of the window, go anywhere and select the sender teleporter. Then go inside of it and... voila, you're inside the window!

You need to kill your character to get out...