Hi Ajay,
Following logic is used to determine the default Investigation Lead when you save the application after selecting Investigation required.
1. Sytem will consider the login user as default investigation lead, but the Login user id will be assigned to Investigation Lead only when below conditions met
a. Login user is provided with "SAP_EHSM_HSS_INCIDENT_MANAGER" role
b. The Role "SAP_EHSM_HSS_INCIDENT_MANAGER" is assigned to the Incident Investigation Workflow(TS500118).
Above BADI is required to overwrite the default investigation lead, if there is no logic to overwrite then it will get the default investigation lead.
Thanks
Sagar Pilli