'Method not allowed' adding subscriber to workitem via POST

I'm trying to write a python script to add a set of subscribers to a workitem doing a POST request with RTC 3.0.1.1.

URL: https://jazz04:9443/jazz/oslc/workitems/9352/rtc_cm:subscr
ibers
Data: {"rdf:resource": "https://jazz:9443/jazz/oslc/users/_n0qiEMI0Ed6zsIiu3BscPw"}
Content-Type: application/x-oslc-cm-change-request+json

This fails with: HTTP Error 405: Method Not Allowed

I've seen articles that suggest this should work:
https://jazz.net/wiki/bin/view/Main/ResourceOrientedWorkItemAPIv2#Adding_Links_to_a_Work_Item
https://jazz.net/forums/viewtopic.php?p=68820

Any thoughts as to what I am doing wrong?

John