Creating a Booking Website with LoginCodes

Hello fellow Programmers,

i just finished my University Degree and got my first Job. My Client wants a Appointment Bookingsystem for his webiste, where the people get a code beforehand and can login with that code to get acces to a specific date. So eg code ABCD enter make the person get acces to book a session an day X and only that should be shown. The website itself is made via Wordpress and i know there are a lot of free booking plugins already, but i havent seen anything which comes close to the code login checks. Is there anything existing already? Do i need to create it all by myself? My approach would than be using PHP for a login and construct a databse with mysql. Isnt that a little overkill? Although im new to Webdevelopment. Thanks already for you replies :)