I assume you are trying to implement "leaver" workflow. In this case, you have to set employeeLeaveDateTime property using Microsoft Graph (or PowerShell) as described here:
https://learn.microsoft.com/en-us/graph/tutorial-lifecycle-workflows-set-employeeleavedatetime?tabs=http
Then you can use employeeLeaveDateTime attribute in the trigger configuration:
https://learn.microsoft.com/en-us/graph/api/resources/identitygovernance-lifecycleworkflows-overview?view=graph-rest-1.0#execution-conditions
Please let me know if this helps.