It is not possible to enforce a specific payment method via a payment link created from the Merchant Back Office.

However, there are two alternative solutions:

• Open a new store
  By opening a new store, you will be able to configure a single payment method for that store. You can then send a payment link via that store.
  To open a new store, contact the E-Banking Merchant Support Service.
• Use WebServices
  Integrate the Charge/CreatePaymentOrder method from WebServices by configuring the paymentMethods field to enforce the desired payment method.
  The WebServices REST feature must be enabled on your store to be used.