There are three ways to generate an access token.
Programmatically using your key—Load your private key, generate an assertion, and call the API to get an access token. You must monitor when the token expires and generate a new one.
Programmatically using a refresh token—Load your private key, generate an assertion, and call the API to get a refresh token. Use that refresh token from then on call the API and generate an access token.
Updated 4 minutes ago