You need another location where the category codes are stored, as the combobox only shows the category names. There are many ways of doing this, and you may already have it.

Presuming that you create a 2nd combobox (11) with the category codes in it, and that the codes match up with the category names in combobox (10), then you should use something like
Code:
on *:dialog:connx:sclick:10:{
  _list $did(11,$did(10).sel)
}


If you have the codes stored in another manner, post how they are stored, and this code can be re-written.