Answer
Read the entity first (to get its current ETag/state) before the Update call, then in the subsequent Update task, pass both the path parameter values and the regular update parameters together in the same call, rather than trying to configure a separate distinct 'path parameter' input the way Postman's UI exposes one. Workflows don't have a dedicated path-parameter field the way Postman does, but combining everything (path-identifying values plus the actual fields being updated) into the Update task's parameters, after a preceding Read, gets the same result.