I needed to use the web form because I extended my visitor status. So there's a part of the form where you can upload a document to send the proof you need.
Hope that helps.
I only needed the UCI and I explained I haven't received my application number yet. But they also ask for our full name and our date of birth.
Here it goes the link: http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=52&top=34
But to apply for an eTA, we need to have a valid American visa and/or have had, in the past 10 years, any kind of Canadian visa issued.
Hope that helps! : )