Module Main
    Friend f1 as New Form1()
    Friend f2 as New Form2()
 
    Sub Main
        f1.ShowDialog()
    End Sub
End Module

Private Sub btn_SearchKfz_Click(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles btn_SearchKfz.Click
         f1.TextBox1.Text = f2.ListBox1.SelectedItem
         Me.Close()
End Sub