You need to have myCIC/GC Key accunt (you might have it if you applied for any permits online before or you can create it).
Then once you logged in there should be a link saying "‘Add (link) your application to your account’". You click on it and choose: Permanent Residency->Family Class(Spouse)/Common Law.
Then you need either UCI # or date of birth/full name of applicant (not sponsor) + passport/birth place.
And then you enter place where you will be living in Canada (as per application) and put marital status + number of applicants (2 - sponsor and applicant).
I was able to find my application and link it to account. I used my full name, date of birth and passport information.