Mattermost request not being parsed. Error: raise InvalidUsage("Failed when parsing body as json"

I am trying to add matter most as the bot platform for RASA. Meanwhile, my request from matter most is


And Iam getting error:

My matter most version is: 5.15.1

Apparently, this issue was due to content type variation while creating outgoing webhooks in mattermost. We had to select Content Type as application/json.