What is the token_auth and where can I find it to use in the API calls?
Token_auth is an encoded version of your username and password which can be used for authenticating all your API requests. It should be handled very carefully and not shared with anyone since every Piwik PRO user has a different one.
To find it just click on the left upper corner Menu and choose the Analytics Settings. You will see the API options on the left menu column, just like below: