access control source property

I'm developing an ActiveX control (my first) intended to be used primarily on MS Access forms. The biggest component of the control is a RichTextBox. I'd like the ActiveX control, when placed on an Access form, to display a Control Source property in which to identify the field containing the RichText for the box on the control. When you place a standard RichTextBox on an Access form, there is a Control Source property available. Anyone know what to do to set this up?

Thanks!
Douglas