Hi Kavi,
The Linked BOR object to MB21 is BUS2093 but there is not event defined in this BOR. So looks like you have to create a Subtype ZBUS2093 of BUS2093 and add a "created" event. Then you can trigger the workflow by triggering this event (using FM SAP_WAPI_CREATE_EVENT) in a User exit or implicit enhancement . Refer to link below
Generic Object Services for MB21
For WF
Release Strategy/Workflow for MB21 Reservation
Regards
Sandy
