When creating a new Lead Capture form you can choose to send submissions directly to your ActiveCampaign account.
Step 1 - Add A New Provider
Visit the 'email integrations' section of your account settings and click the 'add a new provider button'
Choose ActiveCampaign from the list and add your API Key along with your API URL and click 'add integration':
For details on how to find your API settings please read this article.
Step 3a - Lead Capture Select Your List
When someone opts-in to your form their details will be added to an ActiveCampaign list of your choice.
Step 3b: Resource Library - Select A List
If you are creating a Resource Library then you can select which ActiveCampaign list each subscriber should be added to via the 'Optin Settings' screen: