Thank you for your input.
I did the following in GET_DEFINITION:
ls_component-name = 'LAYOUTS'.
ls_component-type ?= cl_abap_typedescr=>describe_by_name( 'ZPMR_S_LAYOUT_DROPDOWN' ). "ls_keyfield-value ).
INSERT ls_component INTO TABLE lt_component.
IF lt_component IS NOT INITIAL.
eo_field_catalog ?= cl_abap_structdescr=>create( p_components = lt_component
p_strict = space ) .
ENDIF.
Later I define the actions for the controls:
ls_action-id = 'ON_DROP_LAYOUTS'.
ls_action-enabled = abap_true.
APPEND ls_action TO et_action_definition.