JSFiddle - React, Tailwind, and code Playground

HTML

<div id="button"><a href="#popup">Pesan</a></div>
    
    <div id="popup">
    	<div class="window">
        	<a href="#" class="close-button" title="Close">X</a>
            <h2>Selamat Datang di DUMET School</h2>
        </div>
    </div>

CSS

* {margin:0; padding: 0;}

body {font-family: Arial, Helvetica, sans-serif;}

/* Tombol Button Pesan */
#button {margin: 5% auto; width: 100px; text-align: center;}
#button a {
	width: 100px;
	height: 30px;
	vertical-align: middle;
	background-color: #F00;
	color: #fff;
	text-decoration: none;
	padding: 10px;
	border-radius: 5px;
	border: 1px solid transparent;
}

/* Jendela Pop Up */
#popup {
	width: 100%;
	height: 100%;
	position: fixed;
	background: rgba(0,0,0,.7);
	top: 0;
	left: 0;
	z-index: 9999;
	visibility: hidden;
}

.window {
	width: 400px;
	height: 100px;
	background: #fff;
	border-radius: 10px;
	position: relative;
	padding: 10px;
	text-align: center;
	margin: 15% auto;
}
.window h2 {
	margin: 30px 0 0 0;
}
/* Button Close */
.close-button {
	width: 6%;
	height: 20%;
	line-height: 23px;
	background: #000;
	border-radius: 50%;
	border: 3px solid #fff;
	display: block;
	text-align: center;
	color: #fff;
	text-decoration: none;
	position: absolute;
	top: -10px;
	right: -10px;	
}

/* Memunculkan Jendela Pop Up*/
#popup:target {
	visibility: visible;
}