Give the user an option to save it as a draft. Fantastical does this, not sure about Apple's own calendar.
They both then show a ‘discard/keep editing’ prompt, but only fantastical has the extra option of a draft.
But in addition to that — fantastical’s popup isn’t even modal. You can push the sheet down to see and interact with the main calendar.
Unfortunately, fantastical doesn’t show the ‘drag handle’ that Mail and other apps use to show this functionality, so it’s not particularly discoverable.