The return Airline ticket is just one piece of evidence they will use to determine whetger or not they believe you will return to your home country, and not stay in Canada. However, it is a very big piece of evidence. If you do not have that, you will need to make sure you have lots of other evidence (Job, assets, family..) in your home country. So you can still apply without the ticket, but it does remove some of that proof.
There are some other things you can do that might be helpful. As the prior person noted, creating a detailed itinerary showing the times and dates you intended to visit is good. You can book a hotel and car rental which can be cancelled easier than an airline ticket. You could also put a note that the purchase of airline ticket is pending VISA approval.
Another thing is that you could possibly buy a FULL REFUNDABLE airline ticket. These are usually more expensive, but in the case you are denied, you can get your money back.
I submitted an itinerary given by a travel agency and the flight reservation, hope that can satisfy the officers.
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