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

<?php
/**
 * EditableEmailField
 *
 * Allow users to define a validating editable email field for a UserDefinedForm
 *
 * @package userforms
 */

class EditableEmailField extends EditableFormField {

	private static $singular_name = 'Email Field';

	private static $plural_name = 'Email Fields';

	private static $db = array(
		'Placeholder' => 'Varchar(255)'
	);

	public function getCMSFields() {
		$this->beforeUpdateCMSFields(function($fields) {
			$fields->addFieldToTab(
				'Root.Main',
				TextField::create(
					'Placeholder',
					_t('EditableTextField.PLACEHOLDER', 'Placeholder')
				)
			);
		});

		return parent::getCMSFields();
	}

	public function getSetsOwnError() {
		return true;
	}

	public function getFormField() {
		$field = EmailField::create($this->Name, $this->EscapedTitle, $this->Default)
			->setFieldHolderTemplate('UserFormsField_holder')
			->setTemplate('UserFormsField');

		$this->doUpdateFormField($field);

		return $field;
	}

	/**
	 * Updates a formfield with the additional metadata specified by this field
	 *
	 * @param FormField $field
	 */
	protected function updateFormField($field) {
		parent::updateFormField($field);

		$field->setAttribute('data-rule-email', true);

		if($this->Placeholder) {
			$field->setAttribute('placeholder', $this->Placeholder);
		}
	}
}