silverstripe-userforms/code/model/editableformfields/EditableDropdown.php

<?php
/**
 * EditableDropdown
 *
 * Represents a modifiable dropdown (select) box on a form
 *
 * @package userforms
 */

class EditableDropdown extends EditableMultipleOptionField {
	
	private static $singular_name = 'Dropdown Field';
	
	private static $plural_name = 'Dropdowns';

	/**
	 * @return FieldList
	 */
	public function getCMSFields() {
		$fields = parent::getCMSFields();

		$fields->removeByName('Default');

		return $fields;
	}
	
	/**
	 * @return DropdownField
	 */
	public function getFormField() {
		$field = DropdownField::create($this->Name, $this->EscapedTitle, $this->getOptionsMap());

		// Set default
		$defaultOption = $this->getDefaultOptions()->first();
		if($defaultOption) {
			$field->setValue($defaultOption->EscapedTitle);
		}
		$this->doUpdateFormField($field);
		return $field;
	}

	public function getSelectorField(EditableCustomRule $rule, $forOnLoad = false) {
		return "$(\"select[name='{$this->Name}']\")";
	}
}