Thank you for your interest in RadControls for Windows 8 HTML.
You can fully customize RadScheduler's event editor by providing a template to the editable.template property. The template can be set as a string, a WinJS.Binding.Template or a function returning the template. Detailed information and code samples of the templates that can be used with RadControls can be found in this article: RadControls Templates.
All of the properties from the event model are available for editing in a customized edit form and you are free to put any HTML elements or controls in the editor template. The only rule is that there must be an input that contains the corresponding event property value. A full list of the event properties can be found in this article: RadScheduler Data-Binding.
There are just two important rules you need to follow to produce a working event editor:
The name attribute of the input element should match the name of the property you want to bind it to.
You need to ensure that the input element value is in the correct format for the given property. For example, the input elements for the Date properties need to be in format YYYY-MM-DDTHH:mm:ss.sssZ.
I have attached an example project of a basic editor customization using WinJS.Binding.Template. The user can edit the title, description and start and end dates for the events. Note that the example uses text input elements for the date values and lacks data validation. In a real scenario you should use UI controls for date and time input and merge the selected values into a hidden input with the corresponding name attribute value.
I hope this information is helpful. We are currently working on documenting this feature. The article with detailed explanations and code samples will be available shortly. In the meantime, if you have any questions or need further assistance, do not hesitate to ask.