The path of the main page is '/jsonapi'.
First, You have to configure permissions.

You MUST confire settings in next section

Headers

To send JSON

Content-type: application/vnd.api+json

To declare JSON reception preference

Accept: application/vnd.api+json

Get authentification token

/oauth/token

Put in headers: 'Accept': 'application/vnd.api+json'

Request in POST mode with in headers: username, password, grant_type ('client_credentials'), client_id, client_secret

JSON response body will contain token type, access token and refresh token

This example module will not use 'refresh token'

Requests used

All forms buttons are accompanied with the request example

Details

RequestFactory.php forge request, executed in UsageJsonApiTrait, these 2 files contain everything you need.