There is no argument to pass when you call this function, simply attach your USDN payment, make sure it is multiple of 0.5 and you will automatically receive the Certification Credits on the address that sent the USDN.
You can use any of SIGN's features with the SIGN utility token based on the daily updated Certification price, this price is calculated to keep the 0.5$ Certification cost stable despite the possible up and down movement of the SIGN token price. The price is updated every 24 hours. You will have to attach the correct payment in SIGN tokens to every Certification aside of the correct transaction fee, in Waves or in SIGN tokens.
Alternatively you can acquire Certification Credits using fiat (paypal or credit card) or USDN. This option allows you to use the SIGN webapp without the need to hold any tokens in your Waves Account, neither SIGN or Waves are required and the transaction fees are paid by the smart contract.
This second feature was implemented to ease mass adoption and ease the process to any user not familiar with cryptocurrency.