Invitation Codes with Membership2

I am setting up Membership 2 pro from scratch. My membership is free but is controlled by an invitation code.

I have added an invitation code for my membership type, and specified on the membership type (under access options) that the membership type requires an invitation code.

I was expecting an input field for the activation code to appear on the the membership registration form, but no such thing.

Am I misunderstanding how this is supposed to work?