http://localhost:3978/api/messages request body

[TAETAEHO]

I wanna test it by using POST API
I started app.py file and I turned on localhost:3978
But when I tried to send POST API, I got this error
'raise DeserializationError("Cannot deserialize content-type: {}".format(content_type))
msrest.exceptions.DeserializationError: Cannot deserialize content-type: text/plain'

I think my request-body form is wrong.
I referred to the link bleow. But It didn't work
https://learn.microsoft.com/en-us/azure/bot-service/rest-api/bot-framework-rest-connector-api-reference?view=azure-bot-service-4.0
Can I get some request-body example??
Plz inform me if anybody knows about it