Hawk uses a metered pricing model, in which certain features consume credits. For a list of those features, see here. This article explains how to purchase credits in order to use those features.
Navigate to Account Settings > Billing
First, make sure that you have entered a valid payment method. Check the information in the Payment Method column for correctness. If you need to enter or update payment information, click the Update Payment Method button.
Update Payment Method
Hawk works with Stripe, a worldwide payment processing leader, to handle payments. Enter your credit card information in the window.
To add minutes to your account, click the Recharge Account button in the upper right.
Select how many minutes you want to purchase. You can use the Custom Quantity box to enter a specific number of minutes. Then click Complete Purchase.
Look in the Current Activity section. You should see a new entry corresponding to the purchase you just made.