You will have to pay 1 dollar per year. It is not much, we know, but if you can trick the system easily to extend your trial account easily, firstly you will first have to delete your WhatsApp account and keep in your mind that you will want to do this when you are a few days away from ending your trial period. To do this you will have to go in WhatsApp Settings=>account=>Delete My Account. After that, enter your mobile number and delete your WhatsApp account. After that create a new account on WhatsApp, following the same steps as when you firstly created your account. After that, you can notice that your trial period has been extended for 1 year.