Login to your SendinBlue account
Click on your username on top right
Click SMTP & API in the drop down
You will see existing API Keys on this page. If you don’t want to use an existing API Key,
Click on Create new API Key button
Enter the API Key name as Opt-in Samurai
Click Generate button
Copy the new API Key
Go to your WP-Admin > Opt-in Samurai > 3rd Party Integration
Click Activate button under SendinBlue logo
Enter or paste the API key and Save Changes