ext.TestExtension.js

( function ( mw, $, OO ) {
	var field = OO.ui.infuse( 'checkfield-box-field' ),
		toggleSwitch2 = new OO.ui.ToggleSwitchWidget( {
		value: field.getValue()
	} );
	// Create a FieldsetLayout to layout and label switches
	var fieldset = new OO.ui.FieldsetLayout( {
		label: 'Toggle switches'
	} );
	fieldset.addItems( [
		new OO.ui.FieldLayout( toggleSwitch2, { label: 'On', align: 'top' } )
	] );
	$( '#checkfield-box-field' ).append( fieldset.$element );
	field.toggle();
}( mediaWiki, jQuery, OO ) );

SpecialTest.php

<?php

class SpecialTest extends FormSpecialPage {
	public function __construct() {
		parent::__construct( 'Test' );
	}

	/**
	 * Get an HTMLForm descriptor array
	 * @return array
	 */
	protected function getFormFields() : array {
		$this->getOutput()->addModules( 'ext.TestExtension' );

		return [
			'check' => [
				'type' => 'check',
				'label' => 'check',
				'id' => 'checkfield-box-field'
			]
		];
	}

	/**
	 * Process the form on POST submission.
	 * @param array $data
	 * @param HTMLForm $form
	 * @return bool|string|array|Status As documented for HTMLForm::trySubmit.
	 */
	public function onSubmit( array $data, HTMLForm $form = null ) {
		wfVarDump( $data );
		return true;
	}
}