Unfortunately, you can't use the same API key for multiple websites.
When you want to use our payment system for another website, you should inform us about this. You can do this by creating a separate website profile for each website. As soon as we have approved the website, you will receive a new API key from us. You can find this API key in your Mollie Dashboard under Developers.
The advantage of this working method is that it's very easy to distinguish between the transactions from the different websites. It allows you to view the statistics per website profile, which makes it easier to enter them into your accounting system.