When creating a new Lead Capture form you can choose to send submissions directly to your CampaignMonitor account.
Step 1 - Add A New Provider
Under the integrations tab select 'providers' and click the 'add a new provider button'
Step 2 - Connect To CampaignMonitor
Enter your API Key and Client ID as shown below.
Step 3 - Select Your List
When someone opts-in to your form their details will be added to a CampaignMonitor list of your choice.