It is possible to use a combination of API calls: CreateProcess, WaitForSingleObject, and CloseHandle.

When a process, say Excel.exe is created and running, then the WaitForSingleObject is called with the process handle and INFINITE flag.

FoxPro is effectively locked until the Excel session is closed. After that just call CloseHandle function.

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And by the way, did you try VBScript object?
cmdReturn = sll.Run(lcCmd, 1, .T.)