You can add a copy of your boarding pass on your receipt page, during/after the registration of your receipt. You may also upload a copy of your flight itinerary instead.
Make sure your name and date of exit are clearly visible.
If your receipt has been rejected because of an issue with your boarding pass, go to the receipt in your account and click the blue button 're-open receipt for submission'. You will then be able to upload a copy of your boarding pass to your receipt.