How to get current active sub process instance id

Hi Team,
I have a workflow, which has subprocesses, there are APIs to completed the User tasks, in case of sub process process instance key will change, how can I get process instance key of active sub process, in order to get the User task of sub process. In C7, this was not an issue since business key would be same for sub process and we were able to get details based on business key.

In Zeebe how can I get the details of user tasks of sub process from parent process instance key.

Thanks!

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