i try to use findwindow & postmessage API function but error message occur "error calling external function findwindow and postmessage" when run the script
Error calling external function generally means that you declared them wrong. One thing you have to watch out for is whether you are using the ANSI or Unicode versions of the functions. I think PB7 can only use the ANSI versions. (There's only a difference if there are string arguments.) The ANSI version generally has "A" on the end of its name. So, for instance, the correct ANSI external function declarations for FindWindow is:
FUNCTION ulong FindWindow(string ClassName,string WinName) LIBRARY "user32.dll" alias for "FindWindowA"
Assuming you have VisualStudio tools installed, you can check which name variants exist of a function by doing "dumpbin /exports user32.dll" etc., and searching the results for the desired function name.