If you would like to add a payment method for your account in the payment portal, you will have to authenticate the payment method you would like to use before being able to request your withdrawal.
Your payment method is already authenticated in a case where you have already made a deposit with the payment method you are going to make a withdrawal to. It is then shown under saved methods.
Under 'Add new method' you will be able to select one of the available payment methods and then 'Verify method'. During the authentication process, a small transaction will be made in order to finalize the verification, that amount will then be added to your balance and be automatically included in your withdrawal request. Therefore, it is important that you do have some balance on your payment method when activating it.
Once the payment method is verified, you will be able to proceed with your withdrawal request.
If you have any further questions regarding this, please contact support.