You can download the full API collection here:
https://github.com/alexandreelise/j4x-api-collection thanks to Alexandre Elise.
This will import directly into Postman which will enable you to use the API simply by adding your key and URL to the collection.
To create a key you need to:
Make sure the plugins are enabled.
Make sure your user has a token
You should include a user token with a bearer authentication header. You can get the token from your super admin account (Joomla API Token tab), given that the relevant plugin (User - Joomla API Token) is enabled.
For the shake of simplicity you can try a curl tool like postman where all the headers can be added from a GUI.
Also as mentioned by Sharky the ...