1. Create an Autoaddress account
Follow this link to navigate to the Account Center sign-up page and enter your details, no credit card required.
https://account.autoaddress.com/signup/
2. Create a BigCommerce Integration
Navigate to the Integrations page and create a new BigCommerce Integration
3. Paste API key into BigCommerce
Retrieve the API key from this new Integration, go back to the Autoaddress app on your BigCommerce store and paste this key into the blank field.