Hi Alex,
First you set the field display type "Drop Down" in component configuration.
After that you need to fill fixed values for the field in CT_FIELD_USAGE parameter of the GET_DATA mehod and also set the fixed value changed flag to abap_true.
Note : Don't forget to set EV_DATA_CHANGED to abap_true in GET_DATA method.
Thanks & Regards
Praveen Gupta