I was trying to add a User Property to an item, NOT a folder. Here is
what I did:

1) Open the custom form.
2) Click on the "All Fields" page
3) Choose "User defined fields in this item"
4) Click on New, and select the type I want.

However, the new user property appeared in the "User defined fields in
Inbox" instead of "User defined fields in this item", and that's not
what I want. What can I do other than using the
Item.UserProeprties.Add method?