Embed a report using html and JS

Hello,

I followed the steps here:

https://docs.microsoft.com/en-us/power-bi/developer/embedded/embed-sample-for-customers

Move your Power BI embedded analytics application to production - Power BI | Microsoft Docs

Including creating an app, workspace, granting all required access, creating an Azure capacity and associating it with the workspace. Now I'm completely lost on how to embed a report using HTML/JS.

There is a sample code here:

Solved: How to Hide Power BI iFrame Embedded Report Tabs - Microsoft Power BI Community

However, I don't know how to create this (accessToken). I believe this is an "Embed Token"? 
Could someone please explain how to get this accessToken using the API, preferably using Postman? 

Thanks