lr_core = cl_crm_bol_core=>get_instance( ).
lr_message_cont = lr_core->get_global_message_cont( ).
lr_message_cont->add_message( iv_msg_type = if_genil_message_container=>mt_error
iv_msg_id = 'CRM_BOL'
iv_msg_number = '017'
iv_msg_v1 = lv_msg_v1
iv_show_only_once = abap_true ).
Here iv_msg_id is Message class which u have created in SE91.
iv_msg_number is the Message Number which u need to display in UI
iv_msg_v1 is the values u need to display in UI,in this case it is Item number.
Regards,
Gayathri A.