Manual:NamespaceInputWidget

NamespaceInputWidget is a Namespace input widget which is used to select a wiki namespace. It is based on OOUI's DropdownInputWidget. but provide wiki namespace as options.

Javasciprt Examples
Using the widget in MediaWiki Extension will be done through the 3 steps. First create the resource javascript file, Second register the js file in extension.json file. So that the module can load by ResourceLoader. And finally load it by Output Context. To use the widget in UserScript is very easy, You just need to load the dependencie by mw.loader.using. and put the code into the block. Try the below code in your Special:MyPage/common.js
 * In MediaWiki Extension
 * Create the resource.js file in extension directory with following code.
 * Register the js file as module in extension.json
 * Finally load it by Output Context in your SpecialPage/ParserHook.
 * In UserScript

To use in gadgets, you have to add a mediawiki.widgets entry in the  field of gadget description. See Gadgets' documentation for instructions and examples.
 * In Gadget
 * So Add dependencies in MediaWiki:Gadgets-definition
 * Create MediaWiki:Gadget-mygadget.js page with code. You does not need to use mw.loader.using as we did in UserScript.

PHP
To use NamespaceInputWidget in MediaWiki Extension's Special Page. You need to decleare the use of MediaWiki\Widget\NamespaceInputWidget. And have to enable OOUI in SpecialPage.