You want an API to manage business roles assign to business user in S/4HANA Cloud.
For the Communication Scenario: Identity Management Integration (SAP_COM_0093) you could refer to API QUERYBUSINESSUSERIN and MANAGEBUSINESSUSERIN.
In regards to managing business roles itself (assign or withdraw catalogs), there is no SOAP available. Currently, this is only in the S/4 HANA Cloud Apps directly possible.
There is a possibility to manage this later on by the IAG tool (using SAP_COM_066 that is only available for customers who purchase an IAG subscription) in the future as one further possibility besides the normal way via the Cloud Apps.
2646803 - How to maintain employees data in an S/4HANA Cloud
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