Gadget-sandbox.js

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$(document).ready(function() {
	
	if (mw.config.get('wgPageName') === 'User:Domino/Sandbox') {
		
		mw.loader.using( 'oojs-ui-core' ).done( function () {
			$( function () {
				var button = new OO.ui.ButtonWidget( {
					label: 'Click me!'
				});
				button.on( 'click', function () {
					alert( 'You clicked the button!' );
				});
				$( '#mw-content-text' ).append( button.$element );
			});
		});
		
	}
	
});

MediaWiki

Gadget-sandbox.js