IMM 5476 is USE of Rep Form , If you are sponsor and want to become a Representative then you need to sign on page 2 and your parents can sign page 3 . If you are unsure then Check out you tube video , there are some really good video.
If you see option to submit in the portal ( GC Key ) then that is the best way to submit otherwise you will have to submit via Webform.
Are you able to connect your application to GCkey ? If yes, then on main page after you log in Click on "Check Full Application Status" Once you are click on it, it will take you to another page in which you will see "View Submitted Application or upload documents" once you are in this page it should show you blue icon "Upload File" , if you are not able to see this option that means you will have to submit it via Webform.