Dynamic Stamp - Setting Dialog Box Defaults

I found how to set the defaults in a stamp dialog box for text boxes, dropdown boxes, and check boxes, but I cannot find out how to set a default for a radio box.

For instance, in the code block below, it's easy to see how the textboxes and dropdown list will have their defaults set. But I have no idea how to do it for the radio boxes. Does anybody know the syntax for this?

var builder = {
    // These map to Text Fields in the Stamp
    textBoxes: [
        { field: "Name", description: "Name:", default: function() { return ""; } },
        { field: "Company Name", description: "Company Name:", default: function() { return ""; } },
    ],
    // This maps to a Popup Group in the PDF named 'Project'
    popupGroup: "Plan Section",
    listItems: [{
        popupItems: {
            //list of items of Popup menu, positive number indicates default selection
            "Civil": +1,
            "Struct": -1,
            "Arch": -1,
            "MEP": -1
        }
    }],
    // This maps to a Radio Group in the PDF named 'Status'
    radioGroup: "Type of Plans",
    radioButtons: [
        // value maps to the 'Choice' of each radio button in the group, description will show on the dialog
        { value: "", description: "Original Plans" },
        { value: "Revised Plans", description: "Revised Plans" },
        { value: "As-Built Plans", description: "As-Built Plans" }
    ],
    radioErrorMsg: "Please select a status"
}