Adding Checkout.fi payment method

Hello.

I’m making a website using wix. It also has services that the customers can buy.The payment method is by Checkout.fi services.

They have given me the security key or the hash key that I need to add in the site so that the payment button would be activated.

Where and how can I add the merchant key and the security key here in the website.

Please, let me know ASAP.

BR.
Srijana KC