Adding extended attribute for an existing forum thread in Javascript

Can someone please suggest the javascript syntax to add an extended attribute for an existing forum thread?

We tried the below code but its not working.

core_v2_forumThread.Update(forumId, threadId, "%{ ExtendedAttributes: _ExtendedAttributes_Test = 'TESTING' }");


However we have tried same with velocity code and it worked.

#set ($forumThreadResponse = $core_v2_forumThread.Update(forumId, threadId, "%{ _ExtendedAttributes_Test = 'Value' }"))