File → Open opens a file dialog that allows you to choose either a .kxf (Koda form) file or .au3 (AutoIt script) file:
Referenced means containing a line beginning
#Region ### START Koda GUI section ### Form=
When Koda generates code, it includes such a line.
Form code can also be extracted from an AutoIt script with Import -> Import AutoIt GUI
The keyboard shortcut for this function is Ctrl+O.