The MailerLite integration is only available if you have a MailerLite account.
Step 1: To find your MailerLite API Key, head to MailerLite Dashboard > Integrations and click Use next to API:
Step 2: Click Generate New Token on the Developer API screen.
Step 3: Name your token something you'll remember, then click Create Token.
Step 4: Copy your new API Key, then navigate to Grow Publisher Portal > Site Settings > Integrations and add your MailerLite API Key:
Step 4: Make sure to click Save Changes once you’ve added the MailerLite API key.