In order to assign a default value to a dialog window field, use the 'default' parameter in the dialog window UI element definition.
elements :
[
{
type : 'text',
id : 'myCustomField',
label : 'My Custom Field',
'default' : 'Default Custom Field Value!'
},
{
type : 'checkbox',
id : 'myCheckbox',
label : 'This checkbox is selected by default',
'default' : true
}
]
The code above creates the following UI elements in a sample dialog window tab.
Sample dialog window tab containing two field with default values
For more examples on setting a default field value refer to the Using the JavaScript API to customize dialog windows (api_dialog.html) sample and its source code from the _samples directory of your CKEditor installation.