Once you select a hosting plan that is eligible for free domain registration, you will see it included in the cart:
You'll be able to register the preferred domain name for free after successfully completing the payment for the hosting plan. To be more specific, you can claim the domain while setting up your hosting plan:
Alternatively, you may do that at any later time from the Members Area of your Hostinger account:
Now you know how to claim the free domain that comes with your new hosting plan.