Fantastical & Mailplane
Fantastical 2.2.5 uses Mailplane to send calendar events when Mailplane is set as your default email application.
Simply go to “Preferences… > General” and enable “Make Mailplane default Mail Application”. And make sure to use at least Mailplane 3.6.5 and Fantastical 2.2.5.
See also their release notes here.