ProximityReader PaymentCardRaeder.Token help

Use case:  Use Apple Tap to Pay API to capture the card details and send the captured encrypted card details to end PSP by using our backend.

We are following Apple document here https://developer.apple.com/documentation/proximityreader/paymentcardreader/token, its mention here that your payment service provider supplies the string you use to create this token. Below is screenshot for your reference:     

Please guide us how we are going to get above mentioned string to create the Token. Any help would be appreciated, thanks in advance