Custom Verification field

Hi,

I’m trying to use different Hook on Ninja forms to make a custom validation field :
In my form, I have a field where user have to enter his Discord ID. During the submission, I would like to check if this user is in server but I don’t find the right hook to make that.
All discord’s ID is in array and I just want to check if the ID is in this array. If it’s yes, the form is save, else a message appear who say : “This user is not in us server”.