Validation Hook for custom Forms possible?

Hello,

I am currently looking into buying the Plugin. I would love to add a “voucher code” field for the user to enter when trying to make a reservation. Then I want to validate this code with some sort of SQL-Statement against the database. Is something like this possible in any way? I am able to code but do not want to change the plugin itself to add these kinds of features. Perhaps there is a way I can use a hook for this kind of validation?

Best regards
Manuel

https://ww.wp.xz.cn/plugins/booking-system/