How to access Outlook "Message" tab from custom form region?

I have a "Separate" form region that is displayed by the IMP.Note message class. I have a button on the custom form that when clicked needs to modify the contents of the Message开发者_JAVA技巧 TextBox on the "Message" tab. How can my custom form region access that TextBox?

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How to: Access the Outlook Item that Displays the Form Region

http://msdn.microsoft.com/en-us/library/bb608615.aspx