If you haven't already, you'll need to sign up for a MailerLite account.
Setting up your MailerLite API key
Navigate to the Developer API screen, by clicking your profile picture and selecting "Integrations". Next, click 'Use' on the Developer API section. (or click this shortcut)
Paste in the API Key on your settings page under "Integrations"
You're all set!
💡 Tip: You can now head to the opt-in form support article to add/complete your opt-in form.