adding the entity primary key to a form

I have created a new entity and I am trying to add the primary key to a new form of that entity but despite me publishing the form and the fields numerous times, its not giving me the option to add the primary key to the form and i cant see it already added. The primary key field is called OrdersID.  I am definitely within the right entity and i can add new fields but i cannot see the primary key field anywhere.

Essentially I am trying to do a form that replicates the "Orders" entity but make it much simpler and useful to my company - I don't want to just edit the "orders" entity, its easier to start from scratch.

I just want it so i can have a field like in Cases / Faults e.g. "CAS-00170-L0B8J9",  but if possible if we could replace the "CAS" with "ORD".

I am using Dynamics CRM 2016 onsite version.

Any help will be greatly appreciated.