Use and integrate my web application user management software in the workflow engin

I have a Java web application with own domain and user management. Now I want to use camunda workflow engine. My question is, how can I use and integrate my web application use management software in the workflow engine?
That means using the own user management service, not camunda…
my environment is spring.
best regards

https://docs.camunda.org/manual/7.8/user-guide/process-engine/identity-service/