InnerException: The remote server returned an error: (400) Bad Request.

Hi Syed_Akhtar,

Thank you for your comprehensive post, and I apologize for the delay in my response. I recommend visiting this help page, which provides troubleshooting steps to resolve this issue: https://developer.myob.com/api/myob-business-api/api-overview/error-messages/  

Feel free to post again, we're happy to help!
Regards,
Earl