From your BigCommerce admin panel, go to 'Advanced Settings' -> 'Legacy API Accounts' to setup your credentials.

Your credentials will be in the following format:

Username: new_user

API Path: https://store-abc123.mybigcommerce.com/api/v2/

API Token: e59d76e03deeb85e0960d982f3d043750d89ecc8

You may also find the following video presentation useful in this context: