This feature seems to be broken 
... you need some API Voodoo magic
Code for PB 12.5:
External Function Declaration:
Functionlong GetWindowLong (ulong hWnd, int nIndex) Library"user32.dll"Aliasfor"GetWindowLongW"
Functionlong SetWindowLong (ulong hWnd, int nIndex, long dwNewLong) Library"user32.dll"Aliasfor"SetWindowLongW"
FunctionLong SetLayeredWindowAttributes(ulong hWnd, Long crKey , byte bAlpha , Long dwFlags) Library"user32.dll"
Open () Event Script in Window:
Constantlong LWA_COLORKEY = 1
Constantlong GWL_EXSTYLE = -20
Constantlong WS_EX_LAYERED = Long(2^19)
SetWindowLong ( Handle(this), GWL_EXSTYLE, &
GetWindowLong( Handle (this), GWL_EXSTYLE ) + WS_EX_LAYERED)
// Make "background" colored pixels transparent
SetLayeredWindowAttributes(Handle(this), This.BackColor , BYTE(0), LWA_COLORKEY)
hth
Arnd