I do not blame the API support team for not wanting to "help". The documentation they have provided is sufficiently well structured for any developer to be able to work it out. This is where you need to start.

 your question seems to indicate you have next to no understanding of the authentication procedure of Oauth2 and I mean no offence to you but I don't blame the API team for not getting involved as they would end up being teachers/tutors rather than doing their own job.

There are literally hundreds of thousands of examples all over the Internet showing how to connect to an API using a OAuth2. . I would suggest you  trying get an understanding of that first as that is by far and away the one thing that confuses most new devs first when connecting to a secured  API

Also, if you're not understanding the actions and communications between a client of the API as you seem to be suggesting below when you say "it means nothing to me" then I think you probably need to do a little bit of research and reading first on how to consume and API

The documentation for the MYOB API is not bad. Try reading it and try starting at the beginning. Then when you do see an example, you'll understand what the code is trying to do. There are no shortcuts to this. You just have to understand the concepts behind how this works. This is not one of those situations where you can just look at a large piece of code and understand exactly what everything does and why it's doing it. For you to understand this, you need to understand the concepts of the access tokens and the role of the client id and cient secret

Authentication (myob.com)

Getting Started (myob.com)

Since myob has a host of POSTMAN assets for you to use why not simply use postman to get an understanding
Authenticate with OAuth 2.0 authentication in Postman | Postman Learning Center

Postman Collection (myob.com)

I was a dev team lead for years and the one thing that seem to always confuse new developers about oauth was the understanding of the access token. Once they understood that they sent to get the whole concept.  here is a great resource on that and if you've read through the postman resources you'll get it as well
Connecting to API using OAuth 2.0 from ALTERYX - The Data School Down Under

Hi ITM 

This is as good a place to start as any

myob-php-api

Cheers

JF