Physical Copy of MCC did not come with a code

My copy of the MCC did not come with a code for Halo Nightfall. How do I get a code and do I need to submit proof of purchase?

No reply? Is there any way I can personally contact 343?

I think you would have to contact your retailer (where you bought the game from) in first instance and ask them about your missing code. They should be able to provide it to you