Is it supposed to be sent in an email? Or is it the reference number on the UChicago website application hub?
Last year it was the reference # you are given once you set up your portal and sign in. You are supposed to refer to it in all communications with admissions and so forth. The Common Ap. will assign you an ID # as well but that’s not the same thing.
https://prospects.uchicago.edu/account/
This should be the link to setup your portal account. You click on forgot password to setup.