Public Class Form1
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles MyBase.Load
        TextBox1.SetBounds(5, 5, 550, 20)
        TextBox1.Text = "Mein Ziel ist es, in dem Text, der in meiner Textbox" & _
          "ist, einen String zu suchen und den dann zu markieren."""
    End Sub
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        TextBox1.Select()
        Dim suchstring As String = "String zu suchen"
        Dim start As Int32 = TextBox1.Text.IndexOf(suchstring)
        If start > -1 Then
            TextBox1.Select(start, suchstring.Length)
        End If
    End Sub
End Class