How to set starUser for each call activity(multi instance)

Hello, I would like to ask how to set the initiator of each sub-process when using Flowable to call activities (multi-instance). I have tried many methods, but all failed. I tried to define it as a variable at the initiator of the start event, but it did not take effect. I also tried using the runtimeService.addUserIdentityLink method, but it also did not take effect.