Service User is a concept of Parallels Plesk Automation (PPA)/Parallels Operations Automation (POA) - they are managed through PBA-S CP for POA/PPA subscription at
PBA-S CP > select a POA/PPA subscription > Users
For details see Parallels Operations Automation 5.5 Subscriber's Guide (Chapter 18).
New Service Users are not propagate to PBA-S, they are able to login though the PPA control panel or POA control panel only.
Such behavior is considered to be a feature request. The fix will be considered and probably implemented in future PBA-S updates. You can track it using the following ID:
PBAS-29365 - Propagate a Service User created in PPA/POA to PBA-S
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