Hi there and thank you for reaching out via the Community forum.
The step you are stuck at is the step where your application must open a new browser window which will host the MYOB Login window for the authorisation.
The redirect portion of this step takes the user back to your application/website once the Authorisation in the separate window has occured. In the backend from this you should recieve an access code which authorises your app to then retrieve an access token & refrsh token and then make calls to the API.
Here is a video showing the full Auth process via the "Postman" application.
https://developer.myob.com/api/accountright/accountright-postman-collection/
And a link to our support document: https://apisupport.myob.com/hc/en-us/articles/360001459455-How-to-Complete-End-User-Authentication-with-OAuth2-0
Retrieving the access code is all that is missing in your steps there. Log into yout MY.MYOB account here: https://my.myob.com.au/Pages/Default.aspx and make sure your redirect URI listed here is the same one you are redirecting back to after auth.
If these links are no extra help please feel free to create an AccountRight API Support ticket here: https://developer.myob.com/contact/
Kind regards
