Hi @Anonymous,
Could you please share a bit more about your scenario?
Do you want to add msg why the event is canceled in your Calendar when you delete the event from your Calendar?
If you want to add msg why the event is canceled in your Calendar when you delete the event from your Calendar, I afraid that there is no direct way to achieve your needs in PowerApps currently.
As an alternative soltuion, you could consider take a try to send an individual email (using Office365.SendEmail() function) to notify the attendees that the corresponding event has been canceled, and you could add related reason msg within the Email body.
I have made a test on my side, please take a try with the following workaround:


Set the Items proeprty of the Calendar Dropdown (Dropdown1) to following:
Office365.CalendarGetTables().value
Set the Items proeprty of the Event Dropdown (Dropdown2) to following:
Office365.CalendarGetItems(Dropdown1.Selected.Name).value
Set the OnSelect property of the "Delete" button to following:
Office365.SendEmail( /* <-- Send Cancel Notification email to the attendees of the event */
Concat(Dropdown2.Selected.Attendees.EmailAddress, EmailAddress.Address & ";"), /* <-- Get related attendees emails of this cancel event*/
Dropdown2.Selected.Subject & " Event Cancel Notification",
ReasonTextInputBox.Text /* <- Type your Cancel reason within the ReasontextInputBox*/
)
Office365.CalendarDeleteItem(Dropdown1.Selected.Name, Dropdown2.Selected.Id); /* Delete Event */
Best regards,
Kris