Step 1: Get your Mailchimp API key
Generate and copy your Mailchimp API key:
Step 2: Get your Audience ID.
Get your Audience ID: https://mailchimp.com/help/find-audience-id.
Step 3: Paste your Mailchimp API key into Run
On the Run form builder, click to add destination, and then enter both your API key and Audience ID. Confirm that it's connecting correctly under Status after you save.
Step 3: Enable Mailchimp on ALL forms you plan to use
On the form builder, you must click on the Enable checkbox to the left of the destination for all forms you want to use this destination for on the site.
In order to add custom fields to your form, open your form builder, click Add Form Field, and add a Custom Field (like 'text' for example). Under the Mailchimp custom fields column, Run will pull down a list of your custom fields from Mailchimp to select from. Typing here will create a new custom field on the Mailchimp side.
You can manage custom fields on the Mailchimp side on the following page. Mailchimp refers to them as Audience Fields and Merge Tags: https://us7.admin.mailchimp.com/lists/settings/merge-tag
Where it is says CUSTOM1 in the image below is the input box that needs to match the field name on the Run form builder.
Advanced and Troubleshooting
Mailchimp has a couple of quirks to keep in mind:
Mailchimp will not save address date like a zip code, unless the entire address is provided. So if you want to make sure address data is saved on Mailchimp, you'll need to include all the address related fields, and set them as required. Alternatively, you could create a new custom field on Mailchimp called Zip Code and link to that instead of their standard address field.
If you do not include the Phone/Email Opt In field on your form or if it's not clicked (you can require it), users who submit their emails will receive a confirmation email from Mailchimp before they're added to your form. We recommend including the Phone/Email Opt In to avoid this step (because that email may go to spam or people may not confirm), and also because it will make you compliant with 10DLC regulations for sending text messages.