Es fácil
Código vb:
Ver originalPrivate Sub RichTextBox1_Click()
Text1.Text = RichTextBox1.SelText
End Sub
Private Sub Text1_KeyUp(KeyCode As Integer, Shift As Integer)
Dim SelSt As Long
SelSt = RichTextBox1.SelStart
RichTextBox1.SelText = Text1.Text
RichTextBox1.SelStart = SelSt
RichTextBox1.SelLength = Len(Text1.Text)
End Sub