I have create a new app in myapps portal but it is not yet submitted for the review. How can I add the App Showdown participation file before send the app to a reviewer? Or do I need to send the app for the review and then I'll be able to fill the Ubuntu App Showdown participation file?
You should upload the participation file itself, instead of a tarball or package for your app. If you've already uploaded something else, just copy/paste the contents of the participation file into the Feedback section of MyApps.