Hello Jeff, and welcome to the Clarizen forums.
A custom field in itself is not mandatory or not.
However, if you choose to create a project from a form, you may create a custom action (at the organization level) to this effect.
In the custom action you may select to add user input, which will create the form itself. In the user input, you can decide what will be mandatory to the creation of the project and what will not. You would then tell the custom action to update your custom field in accordance to what was inputted by the user.
Don't forget to add your custom field to the view/grid and/or project property card.
I invite you to read up on custom actions in our user manual, and hop onto one of our Q&A webinars where we can show you the basics of custom actions.
Best of luck in your use of Clarizen.
Jonathan Angel, Clarizen Customer Success