Imports System.Runtime.InteropServices
 
Public Class Form1
    Private Declare Function SendMessage Lib "user32.dll" Alias "SendMessageA" _
      (ByVal handle As IntPtr, ByVal uMsg As Integer, _
       ByVal wParam As IntPtr, ByVal lParam As String) As IntPtr
 
    Const WM_SETTEXT As Integer = &HC
 
    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
    Private Shared Function FindWindow(ByVal lpClassName As String, ByVal _
    lpWindowTitle As String) As IntPtr
    End Function
 
    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) _
      Handles Button1.Click
        Dim windows As IntPtr = FindWindow(vbNullString, "Administrator:" & _
        "Eingabeaufforderung")
        SendMessage(windows, WM_SETTEXT, 0, TextBox1.Text)
    End Sub
End Class