Integrate the site with the payment system

Site on Tilda you need to add a payment method for this service –
https://doc.sbctech.ru/card_payment_API/sale-transactions.html#payment-form-integration

the problem is as follows, in this payment method there is no API for authorization through js, the first request must in any case be from the backend.
and on the tilde there is no way to work through the backend.
How integration can be performed