Как добавить текст в ComboBox?

ComboBox1.Text = "любой текст"

Public Class Form1
 
  Dim str As String = ""
 
  Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    ComboBox1.Items.Add("") : ComboBox1.Items.Add("Материал")
    ComboBox2.Items.Add("") : ComboBox2.Items.Add("Материал")
  End Sub

  Private Sub ComboBox1_TextChanged(ByVal sender As ComboBox, ByVal e As System.EventArgs) Handles ComboBox1.TextChanged
    TB1.TabPages(TB1.TabPages.IndexOf(sender.Parent)).Text = sender.Text
  End Sub

  Private Sub ComboBox1_MouseDown(ByVal sender As ComboBox, ByVal e As System.Windows.Forms.MouseEventArgs) Handles ComboBox1.MouseDown
    str = "" 'переменная содержит текст из редактируемой части ComboBox1 
    str = sender.Parent.Text
  End Sub
 
  Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As ComboBox, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
    Select Case sender.Text
      Case "Материал"
        If str <> "Материал" Then
          sender.Text = str
          ComboBox2.Text = str
          MsgBox("Поле " + Chr(34) + "Материал" + Chr(34) + "уже назначено.", MsgBoxStyle.Critical)
        End If
    End Select
    TB1.TabPages(TB1.TabPages.IndexOf(sender.Parent)).Text = str
  End Sub

 sender.Text = str

MsgBox(str)

 Dim Str As String = "ABCDEFG"
    Private Sub ComboBox1_SelectedIndexChanged(sender As ComboBox, e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
        Timer1.Interval = 500
        Timer1.Start()
    End Sub
 
    Private Sub Timer1_Tick(sender As System.Object, e As System.EventArgs) Handles Timer1.Tick
        ComboBox1.Text = Str
        Timer1.Stop()
    End Sub