By Follow steps below, you can find the api key or create the api key at mailchimp and use it to your website to connected the site with mailchimp to collect the subscribe in fastest way.
- Login to Mailchimp account
- On upper right side MY ACCOUNT >> Profile, click on profile link
- On Profile Page, under Extra >> API Keys
- Create API Key and a New API will generated or you can find your old keys here.
Below steps with images: