If anyone is familiar with Salesforce Marketing cloud or tried to Onboard to IdentiyNow . The application has SOAP API that can be used to list all users but I did not find anything specific for user management .
I am not familiar with this application myself, but I did some searching and stumbled upon this article: How to Create Users via API in Marketing Cloud | Slalom Technology. It looks like it is possible to create/update users vis the API, so this should be able to be translated into a Web Services connector in ISC. Maybe use this as a guide to figure out the correct endpoints/payload in Postman then transition to an ISC Web Services connector.
your approach did get me a way ahead and I was able to successfully connect to a Salesforce Marketing cloud instance web services SaaS and do a successful test connection operation.