You need to create a completely free level.
This can be done by creating a new button and then enable the submenu option.
When you enter the new sub menu, the screen type selection is enabled until you place a button on that menu.
If you want to change the screen type for an existing level, you need to drag all pressent buttons to the clip board, then change the screen type and drag the buttons back. Be carefull with this, you may have serialisation errors depending where in the configuration you do this.