How To Destroy a MODAL Dialog Box Window From Another Application

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VB

I tried using DestroyWindow to remotely destroy a password prompt dialog box that keeps popping-up every time I remotely open a particular PDF document. (It doesn't work!)

Before this I was also trying SendMessage with the WM_CLOSE value as shown below:

Public Const WM_CLOSE = &H10

Declare Auto Function SendMessage Lib "user32.dll" (ByVal hWnd As IntPtr, ByVal Msg As Integer, _
         ByVal wParam As IntPtr, ByRef lParam As IntPtr) As IntPtr

    Declare Auto Function SendMessage Lib "user32.dll" (ByVal hWnd As IntPtr, ByVal Msg As Integer, _
        ByVal wParam As Integer, ByRef lParam As IntPtr) As Integer

    Declare Auto Function SendMessage Lib "user32.dll" (ByVal hWnd As IntPtr, ByVal Msg As Integer, _
        ByVal wParam As Integer, ByRef lParam As Integer) As Integer

    Declare Auto Function SendMessage Lib "user32.dll" (ByVal hwnd As IntPtr, ByVal wMsg As Integer, _
        ByVal wparam As Integer, ByVal lparam As System.Text.StringBuilder) As IntPtr

Dim dialog_title As String = "Password"
Dim hwnd As IntPtr = FindWindow(vbNullString, dialog_title)
SendMessage(hwnd, WM_CLOSE, 0, 0)

When I do this virtually nothing happens (nothing ever happens).
Upon keen observation I did notice that the cursor blinking in the password input field went off for about half-a-second then started blinking again; that's all.

Maybe I'm not using the SendMessage function right; I dunno. I've tried so many ways.

Can anyone see whats wrong with my code or suggest a "sure-fire" way that might work to get rid of the modal dialog.

Thanks.