I think you just pass the app class rather than finding it, like this. Edit: SystemProperty("AppHWND") does the same thing as FindWindow but doesn't need the names.
.lib "User32.dll" Api_FindWindow%(classname$, windowname$):"FindWindowA"
; Const class$="GX_WIN32_CLASS" ;BlitzPlus 1.11 ; Const class$="BLITZMAX_WINDOW_CLASS" ;BlitzPlus 1.34 Const class$="Blitz Runtime Class" ;Blitz3D, app class Const title$ = "My Blitz Window" ;app title AppTitle title$ hwnd = Api_FindWindow(class$, title$)