How to posts a message in Yammer as a organisation name(not as a user) using Yammer REST API from server-side?

I am trying to send messages through yammer POST API from the server side -> "https://www.yammer.com/api/v1/messages.json"

with headers -> "Authorization" : "Bearer #authToken".

and with body ->

{
       
 "body": "This is a Spot Award",
       
 "group_id": "$Group_id",
    
 "skip_body_notifications": "true",
       
 "is_rich_text": "true",
       
 "message_type": "announcement",
       
 "title": "Spot Award",
       
 "og_description": "Monetary Award",
        
"og_private":"private"

}

But it posts against my user name and I want to send it against my organisation's name. And this is happening due to the #authToken which is generated with user/Application Combination.

How can I send posts against my Organisation's name to any groups?