I checked in Developer Documentation 7.5-7.6 and did not see anything but list Hash Tag REST Endpoint. Is there a api call for add and deleting a tag on a thread?
Have you looked at the REST endpoint for "update forum": telligent.com/.../34980.update-forum-thread-rest-endpoint.aspx
Or, if you are interested in the In-Process API, there's a Tags In-Process API Service: telligent.com/.../34274.tags-in-process-api-service.aspx
Telligent Systems, Inc. ©2013, All Rights Reserved