If you haven't already, you'll need to sign up for an ConvertKit account.
Setting up your ConvertKit API key
Navigate to the Advanced settings screen, by clicking the settings link after clicking your profile image. Next, click 'Advanced' in the left hand menu.
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.