An API Key: View and generate API keys in your Mailjet API Key Management page.
A Secret Key: View your API Secret Keys in your Mailjet API Key Management page.
Optional: Select whether to use Sandbox Mode for calls made using this credential. When turned on, all API calls use Sandbox mode: the API will still validate the payloads but won’t deliver the actual messages. This can be useful to troubleshoot any payload error messages without actually sending messages. Refer to Mailjet’s Sandbox Mode documentation for more information.