If you use a representative, you won't be able to link it online, but you can still use eCAS (
https://services3.cic.gc.ca/ecas/authenticate.do?app=)
Normally, for linking you use the Principal Applicant's UCI (you can't link the sponsor's UCI to an app).
Since you have a representative, they will instead have access to the Paid Representative Portal which is normally a lot more detailed and has a lot fewer glitches and stuff, so if you keep in constant contact with your representative, they should be able to provide you more detailed information as your app progresses which is better than what you would get on MyCIC anyway.