An API Key: Generate an API key from the Integrations menu. Refer to the API Authentication documentation for more detailed instructions.
Enable the Classic API toggle if the API key is for a MailerLite Classic account instead of the newer MailerLite experience.
Most new MailerLite accounts and all free accounts should disable the Classic API toggle. You can find out which version of MailerLite you are using and learn more about the differences between the two in the MailerLite FAQ.