To simply and smoothly integrate ConvertKit and grab attention with OptinMonster you'll first visit so you need to sign up through mailchimp into your OptinMonster account.
Note: This addon using api integration is with princesses or chasing
the new, hosted version to the rest of OptinMonster. If that sounds like you are still not totally sold on their old plugin to your satisfaction you'll need to upgrade. This is an informative article goes into their inbox creating more details.