Excel Interacting With Another Non-Office Application

[Kenneth Hobson]

Obviously, you have posted a snippet. For the FindWindow() API routine to work, it needs to be defined at the top of a Module before other code. That code can vary depending on 32bit or 64bit computers.
e.g. for 32 bit:

Declare Function FindWindow Lib "user32" Alias _
"FindWindowA" (ByVal lpClassName As String, _
                    ByVal lpWindowName As Long) As Long

For SendKeys() to work properly, it that is even possible, be sure that you turn off Windows, UAC.

For a more robust API example:

#If VBA7 Then
    Dim mhwndForm As LongPtr            'The userform's window handle
    Private Declare PtrSafe Function FindWindow32 Lib "USER32" Alias "FindWindowA" (ByVal lpClassName As String, _
    ByVal lpWindowName As String) As LongPtr
    Private Declare PtrSafe Sub SetWindowPos Lib "USER32" (ByVal hwnd As LongPtr, ByVal hWndInsertAfter As LongPtr, _
    ByVal X As Long, ByVal Y As Long, ByVal cx As Long, _
    ByVal cy As Long, ByVal wFlags As Long)
    Private Const HWND_TOPMOST As LongPtr = -1
    Private Const HWND_NOTOPMOST As LongPtr = -2
#Else
    Dim mhwndForm As Long             'The userform's window handle
    Private Declare Function FindWindow32 Lib "USER32" Alias "FindWindowA" (ByVal lpClassName As String, _
    ByVal lpWindowName As String) As Long
    Private Declare Sub SetWindowPos Lib "USER32" (ByVal hwnd As Long, ByVal hWndInsertAfter As Long, _
    ByVal X As Long, ByVal Y As Long, ByVal cx As Long, _
    ByVal cy As Long, ByVal wFlags As Long)
    Private Const HWND_TOPMOST As Long = -1
    Private Const HWND_NOTOPMOST As Long = -2
#End If
Private Const SWP_NOSIZE             As Long = &H1
Private Const SWP_NOMOVE             As Long = &H2
Private Const SWP_NOACTIVATE             As Long = &H10
Private Const SWP_SHOWWINDOW             As Long = &H40