Web portal - Search field not appears

Hi,

For some of my products, there are requests properties based on foreign keys.
I made custom columns in the ShoppingCartItem and pwo tables (CCC_uid_adsgroup, ccc_uid_person, etc...)

In the web portal, for every product using this kind of property, the user have to clik on "Assign" and, there is a search field to filter results.

This is working fine for every cases, except for one key : ccc_uid_o3emailbox : there is no search field when the user clik on "assign".

Could someone tell me how to make this search field appears ?