Create a hidden text field using user event script and store a default value

const beforeLoad = (scriptContext) => {
    let form = scriptContext.form;
    let redirectUrl = form.addField({
        id: 'custpage_textfield',
        type: serverWidget.FieldType.TEXTAREA,
        label: 'Field Label'
    })
    redirectUrl.updateDisplayType({
        displayType: serverWidget.FieldDisplayType.HIDDEN
    });
    redirectUrl.defaultValue = "Sample Text";
}