The API key is available in your OnceHub account, under ScheduleOnce > Setup > Integrations > API Integration. If you don't have an account, you can sign up for a free trial.
Authenticate each HTTP request you send by passing your API key in the
All API requests must be made over HTTPS. Calls made over plain HTTP will fail. API requests without authentication will also fail.
Keep your API key secret
Your API key carries many privileges, so be sure to keep it secret! Do not share your secret API key in publicly accessible areas such as GitHub, client-side code, and so forth.