I’m currently still investigating this scenario and need more time to provide you with solution, suggestion, or sample.
I’ll get back to you as soon as possible.

[Edit]

How can I set the value on the combo from the webscheduler?

eg.: I would like to open the pop up from the webscheduler and set the default value for the field category on the Page_Load.

There are two pop up of WebScheduler, the WebCallOut control and the editing form. I assume that the pop up on your required scenario is the editing form.

Please follow the step-by-step below in order to set the default value for the field category of WebScheduler’s editing form.

1. Add OnEditingFormInitialize client-side event in WebScheduler.
   The snippet code below shows the code after adding the OnEditingFormInitialize client-side event of WebScheduler.

   <ISWebScheduler:WebScheduler ID="WebScheduler1" runat="server" ...>
       <ClientSideEvents OnEditingFormInitialize="WebScheduler1_OnEditingFormInitialize" />
       ...
   </ISWebScheduler:WebScheduler>

   
   

   function WebScheduler1_OnEditingFormInitialize(controlId, action, eventView, eventType, newType) {
       var WebScheduler1 = ISGetObject(controlId);
   
       return true;
   }

   
   
2. Access the editing form window object using the GetEditingFormWindow() method.
   

   function WebScheduler1_OnEditingFormInitialize(controlId, action, eventView, eventType, newType) {
       var WebScheduler1 = ISGetObject(controlId);
       var editingForm = WebScheduler1.GetEditingFormWindow();
       var wcCategories = editingForm.ISGetObject("wcCategories");
   
       return true;
   }

   
   
3. Set the desired selected row of WebCombo.
   

   function WebScheduler1_OnEditingFormInitialize(controlId, action, eventView, eventType, newType) {
       var WebScheduler1 = ISGetObject(controlId);
       var editingForm = WebScheduler1.GetEditingFormWindow();
       var wcCategories = editingForm.ISGetObject("wcCategories");
   
       wcCategories.SetSelectedIndex(2);
   
       return true;
   }

   
   

There are four different way to set a value of WebCombo using client-side script.

• Set value directly.

  function ValueDirectly()
  {
      var combo = ISGetObject("WebCombo1");
      combo.SetText("Brian");
      alert("Text set to Brian");
  }

  
  
• Set value by index.

  function ValueIndex()
  {
      var combo = ISGetObject("WebCombo1");
      combo.SetSelectedIndex(2);
      alert("Set value by index 2");
  }

  
  
• Set value by key value.

  function keyValue()
  {
      var combo = ISGetObject("WebCombo1");
      combo.SetSelectedRowByValue("Andrew");
      alert("Set value by KeyValue Andrew");
  }

  
  
• Set value by row index.

  function ValueRow()
  {
      var combo = ISGetObject("WebCombo1");
      var rows = combo.GetRows();
      var row = rows[1];
      row.Select();
      alert("Set value by calling Select() method of a WebComboRow object");
  }

  
  

[Edit]

Part of the same question. How can I use the code behind on the EditingForm popup from the webscheduler?
Every example that I saw tha EditingForm does not have the file .cs

In the current build version of WebScheduler, WebScheduler 3.0, it is not possible to use the code behind on the EditingForm of WebScheduler.

The way how the EditingForm works is described as follow. The data of the selected event is serialized and sent to the client-side. When the EditingForm is invoked, the EditingForm obtained the data of the selected event from client-side and set the default value for each controls.

Hope this helps.