There is no need to create an API key for the BigCommerce integration. If you look at the FAQ they provide, they give you an API key you can copy/paste in. https://support.bigcommerce.com/questions/1518/Integrating+Constant+Contact+with+BigCommerce
Feel free to provide feedback to BigCommerce about this. We've reached out to them a few times about hard coding the API key, there is no reason to copy/paste or have a user enter this as the API key identifies a developer not a customer.