After you install the mollie payment gateway, you will see this settings:
USD Conversion - If you enable this option, the payment amount will be automatically converted to USD. For example, if your system currency is set to Philippine Peso (PHP), it will automatically convert to USD in mollie's payment page. This is specifically useful if your system currency is not supported by mollie because using an unsupported currency can cause issues with the payment gateway. Supported currencies for mollie are here: https://docs.mollie.com/payments/multicurrency
API Key - Your mollie API key.