Hi @670017569,
first, welcome to the Zeebe community 
Usually, user tasks are performed by Tasklist. A user claims a task and completes it using the application.
However, if you really want to complete the job by a client then you should use the same API as for service tasks.
All user tasks have the job type io.camunda.zeebe:userTask as described in the docs.
You could get the jobs for example by using a job worker. Then, use the complete command with the key of the job.
Does this help you?
Best regards,
Philipp