send them a webform with your UCI number (this can be found on your work permit) on that so they can locate and add the additional document to your application.
1- they will add it
2- if the document is important, they might mark your application as incomplete and send it back after 3month
3- if the document is not important, they just do a complete check, give you the AOR with Application number, ask you to link it online, then ask for Additional Document, this time you will submit via GC key online.
I've been there before (missed 1 document as Spouse Background Declaration), they just asked for it. We scanned and sent them online.