We found a common error where developers use their MYOB Client ID/account number in place of their API key and receive Invalid_request error.
In our documentation, client_id is a term that is used to refer to the API key. This is actually the key that you generate in your my.myob Developer Dashboard, when you register your app. You will need to use this client_id/API key for any API communications. for example, to GET access token.
On the other hand, your MYOB Client ID is the 7 to 8 digit number as per below screenshot. So, please make sure to use this API key for any API requests; NOT your MYOB Client ID.
If you have any questions, feel free to log a ticket with the Customisation & Integration Team.