To pay for your Facebook (Meta) ads, you have to be the admin of the ad account. At the same time, you should have a default payment method already set on your account. So, if you already have a payment method set on your account, go to Payment Settings > Amount due > Pay Now > enter the value you want to pay > Select the payment method. But, to pay for the ad with a new payment method, you must first select the one to add. Press Next > add the details of the new payment method > Save > go to Review your payment section > Pay Now > Done.