Where can i add a field in the appointement insert query?

Hi,

I bought the appointements+ plugin, and i want to add a field in the appointement.

(I'm an advanced wordpress user, and i'm not affraid of the code)

I found how to add it in the frontend on the appointement form, but where (in which file) in the plugin can i add it in de database and where can i got it in the admin panel?

Thank you