You will have to add a payment method in order to activate a Microsoft Advertising promotional offer. Depending on the country you used in your account, you will have different payment methods. Usually, these are Credit cards and PayPal. Keep in mind that Microsoft Advertising might make a small $1 charge to your credit card in order to verify it. You can add a payment method by going to your Microsoft Advertising Account > Tools > Billing & Payments > Payment methods.