I cannot control PDFtk through VBA code

see here the VBA code tested; many thanks

Sub Protectpdf()
    Dim PDFtkPath As String
    Dim InputFilePath As String
    Dim OutputFilePath As String
    Dim Password As String
    Dim ShellCmd As String

    ' Path to the PDFtk executable
  PDFtkPath = "C:\Program Files (x86)\PDFtk\bin\Pdftkxp.exe"
    
    ' Path to the input PDF file
    InputFilePath = "D:\Test\input.pdf"
    ' Path to the output PDF file (password protected)
    OutputFilePath = "D:\Test\output.pdf"
    
    
    ' Password for the PDF file
    Password = "090046"

    ' Assemble the command to password protect the PDF using PDFtk
    
    ShellCmd = PDFtkPath & " " & InputFilePath & " output " & OutputFilePath & " user_pw " & Password
    
    'alternative syntax not working:
    'ShellCmd = Chr(34) & PDFtkPath & Chr(34) & " " & Chr(34) & InputFilePath & Chr(34) & " output " & Chr(34) & OutputFilePath & Chr(34) & " user_pw " & Password
        
    'or, another one not working:
    'ShellCmd = PDFtkPath & " """ & InputFilePath & """ output """ & OutputFilePath & """ user_pw " & Password

    
    ' Use the Shell function to execute the PDFtk command
    Shell ShellCmd, vbHide

    MsgBox "PDF file has been password protected and saved to " & OutputFilePath
End Sub