You pop that in the OP when using it ‘per-Reply’ within a Topic with a dedicated calendar - this means that you simply drop a ‘Insert date/time’ into the reply, and it will appear in that calendar. The Staff Calendar is a great example of this.
If you are using it ‘per-Topic’ with a Category (or site-wide) calendar, you add an Event (from the composer + menu) in the OP of each Topic. The option to do so doesn’t appear in replies by design.
The settings are quite confusing (especially which applies to which approach):