@DS
Thank you for posting in Microsoft Q&A forum.
Sorry, I'm not very familiar with the method InitiateClientOperationEx, however I found an article that tell us how to trigger CMPivot through the Cloud Management Gateway, you may check it to see if it helps:
(Third-party article, just for your reference)
https://www.asquaredozen.com/2019/06/18/configmgr-1906-technical-preview-testing-cmpivot-over-cmg-using-adminservice/
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