Choose Field allows you to bind a control on your form to an Outlook property, so that the data the user enters is stored in the item when it's saved. It doesn't have a lot to do with populating a list box, unless you want a static list that doesn't change. That doesn't sound like what you want, but you really haven't sketched out enough of your scenario for me to make concrete suggestions. Did you look at the custom form sample on the page I suggested? Maybe that will give you some context to describe what you want.
Other key issues are what type of form (message, contact, etc.), whether you're using Exchange as your mail server, where you plan to publish the form, etc.
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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers