On form load make sure you set properties: (Also can be done in the properties window)

Code:
Me.txtEmailUser.AutoCompleteSource = AutoCompleteSource.CustomSource
Me.txtEmailUser.AutoCompleteMode = AutoCompleteMode.Suggest
For example when adding a new entry.

vb Code:
  1. Private Sub btnAddNewSuggest_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAddNewSuggest.Click
  2.  
  3. If String.IsNullOrEmpty(Me.txtAddToList.Text) Then
  4.      Exit Sub
  5. End If
  6.  
  7. Me.ListBox1.Items.Add(Me.txtAddToList.Text)
  8. Me.txtAddToList.Clear()
  9. Me.SetCompleteList(Me.txtEmailUser, Me.ListBox1.Items)
  10.  
  11. End Sub