ID of selected value from a Dropdown list

Hello, I made a drop down list to allow the user to choose a year. I can display my dropdown list, but how can I get the ID of the selected value ?

wa_current_year = wa_year_max.
WHILE wa_current_year >= wa_year_min.
wa_listbox-key = wa_current_year.
wa_listbox-text = wa_current_year.
append wa_listbox to it_listbox.
wa_current_year = wa_current_year - 1.
ENDWHILE.

ld_field = 'GV_COMBOBOX_YEAR'.
CALL FUNCTION 'VRM_SET_VALUES'
EXPORTING
id = ld_field
values = it_listbox.