silverstripe-docsviewer/code/forms/DocumentationSearchForm.php

<?php

class DocumentationSearchForm extends Form {

	public function __construct($controller) {
		$fields = new FieldList(
			TextField::create('q', _t('DocumentationViewer.SEARCH', 'Search'), '')
				->setAttribute('placeholder', _t('DocumentationViewer.SEARCH', 'Search'))
		);
		
		$actions = new FieldList(
			new FormAction('results', _t('DocumentationViewer.SEARCH', 'Search'))
		);

		parent::__construct($controller, 'DocumentationSearchForm', $fields, $actions);

		$this->disableSecurityToken();
		$this->setFormMethod('GET');
		
		if($controller->getPage()) {
			$this->setFormAction($controller->getPage()->getEntity()->Link());
		} else {
			$this->setFormAction($controller->Link());
		}

		$this->addExtraClass('search');
	}
}