Sometimes I see events like these on external calendars shown in my Calendar.app:
There is a striped pattern behind it. Makes it look cancelled.
How do I add this effect to events that I create myself?
That is how Calendar indicates that you have responded with a 'Maybe' to the meeting invite. There isn't a way that you can create your own meeting and respond to yourself with a maybe. However, you could create a separate calendar account and send yourself (original calendar) invites, to which you respond 'Maybe'.