REST API - Posting a comment as a specific user


I want to post a reply on a thread via the REST API, end point api.ashx/v2/forums/{forumid}/threads/{threadid}/replies.xml. However the API always posts as the authenticating user e.g. Admin. How can I post as a specific user? I appreciate the API call does not have a username parameter, can someone advise how I can achieve this?

Thanks in advance