The live URL prefix is required for live Adyen accounts. You can find this in your Adyen merchant account.
Then go to Account> API URLs.
Copy the part from the unique number / letter combination, for example: 1797a841fbb37ca7-AdyenDemo.
You need to enter this live URL prefix in the WooCommerce plugin.
Are you stuck? Then you can also view Adyen's documentation: https://docs.adyen.com/development-resources/live-endpoints.