I am looking for solution which is exactly described \HERE
However I am not sure what should I pass to the second parameter ie request_body

digester = hmac.new(secret, request_body, hashlib.sha256) 

I have tried to pass

"request" which throw error "must be convertible to a buffer, not LocalProxy".

"request.httprequest" which throw error "must be convertible to a buffer, not Request".

"request.jsonrequest" which throw error "must be convertible to a buffer, not dict".

"str(request.jsonrequest)" which process this but give invalid signature.

Any input other than above will be very helpful.
If you are not sure but you may have possible solution. Give a try, I will test and let you know.
I am using odoo v10