You can pass "user_name" as part of the "requester" data passed to the API.
See an example here: https://www.manageiq.org/docs/reference/latest/api/examples/provision_request
does this also work with service requests?
sorry to reopen, question was raised by the customer.
Checking the code it does not appear that services supports the same concept. The BZ would need to be reopen if the RFE is still needed.