ClickFunnels is a sales and marketing funnel system that integrates with ConvertKit. You can connect the two using your ConvertKit API key and the options inside of ClickFunnels.
Let's walk through it.
1. Connect the API
First inside ClickFunnels go to Settings -> Integrations. Then click the "+ Add New Integration" button.
Select "ConvertKit Api" from the integration list and then give your integration a nickname.
After saving the integration go back and edit it to see the API Key option. Find your API key inside ConvertKit (on the Account page) and paste it into that field.
2. Setup your funnel
Now with that integration setup you can select it from your integrations list when editing a page in a funnel.
Subscribe to a form is the only action currently available. But then you can select which form you want to add them to.
This works on email opt-in forms as well as sales pages when someone is making a purchase.