API: PropertyManagerPage with SelectionBox

I'm writing an addin which adds a macro feature and then uses a
PropertyManagerPage to edit the macro feature. On this page, I have a
selection box. I can display the property manager page just fine, and
the selection box works, but I can't get it to show an initial value -
it always starts empty. I show the page, set the focus to my selection
box, and then use SelectByID to select an object. The object is
selected but it is not listed in the selection box. If I then actually
click on the same object, I have to click twice - once to unselect it
and then select it again, and then it will show in the selection box.

I'm writing this in C++, but if you can do it in VB, I can adapt it to
C++.

Thanks for your help,
Jonathan Anderson