When you first sign up for Ninja Number, one of the questions we ask is what you want your area code to be. Instead of entering an area code, enter one of the toll-free prefixes (833, 844, 855, 866, 877, or 888), then choose what number you want from the available numbers.
If you're choosing an additional number, click on Billing, then click "Add Number" in the left-hand window.