Private Sub Command1_Click()
    Call TextInRTB(RichTextBox1, "Test", 4)
End Sub
 
Private Sub TextInRTB(RTB As RichTextBox, Einfuegen As String, Pos As Long)
   Dim anfang, ende as String
    With RTB
        .Visible = False
        anfang = Mid$(.Text, 1, Pos)
        ende = Mid$(.Text, Pos + 1, Len(.Text))
        .Text = anfang & Einfuegen & ende
        .Visible = True
    End With
    Exit Sub
End Sub