How to lock non-current customers out of order form with a customer ID

I am looking to set up an order form on my website for an already existing customer base that would be exclusive to those customers. I already have a set amount of customers since I am working with a small produce business and can not have just any person come onto our website and order.

I am hoping that there is a way when I make an order form that the customer has to enter a customer ID that we provide to the customer to make sure unwanted orders do not make it through. Is there any way I could create Customer IDs that are needed to be input by the customer in the order form to ensure that it is an approved customer ordering? 

Thank you!