You can use
Panel.Defaultbutton
set to the Submit button as
Tadit's suggestion.
One more thing you should consider is the setting of Defaultbutton doesn't guarantee only the Submit button is fired when user clicks Enter Key since the behavior is such that when the focus is on other buttons pressing Enter will fire that button instead of default one.
So try to also add a
UseSubmitBehavior="false"
for Cancel button to avoid such behavior.