Auto populate one form field to another in the same form

Hello I'm using the custom fields addon and I have added 1. FirstName, 2. LastName, 3. Username, 4. Display Name and 5. Nick Name fields in that order on the registration form. I want when someone type FirstName and LastName in, for whatever they put in those fields, to automatically populate in Display Name field i.e. FirtName Lastname. They could then just keep that or type over it with whatever they want. Anyone know how to accomplish this?