Hi Marvin,
Thanks for using Syncfusion product.
We don’t have any support to customize the default appointment editor but you can create your own appointment editor. And you can refer sample for the same in the below sample location.
Sample Location: C:\Users\vigneshkumar.ramasam\AppData\Local\Syncfusion\EssentialStudio\13.3.0.1\WPF\SfSchedule.WPF\Samples\CustomizationDemo
Regards
Vigneshkumar R