Is there an API to set values for a Multivalued soft attribute ?
You can use the PersistableAdapter API for getting /setting attributes.
See API document of this API at below location
/Windchill_10.2_2/Windchill/codebase/wt/clients/library/api/com/ptc/core/lwc/server/PersistableAdapter.html
Hope this helps !!
Thanks
Shreyas