Simple WP Ajax

function cb_related_ajax()
{
	$sample_ajax     =     $_POST['hello'];

  	echo $sample_ajax;

	wp_die();
}

add_action('wp_ajax_cb_related_ajax','cb_related_ajax');
add_action('wp_ajax_nopriv_cb_related_ajax','cb_related_ajax');


add_action('wp_head',function(){
	?>
	<script type="text/javascript">
		document.addEventListener('DOMContentLoaded', function() {
		  	// code
		  	
			[].forEach.call(document.querySelectorAll('.cb_relate_btn'), function(el) {
			  el.addEventListener('click', function() {
			    // code…
			    cb_related_ajax();
			  })
			})

		  	//ajax
		  	function cb_related_ajax(){
		  		var ajax_url = window.location.protocol + "//" + window.location.host+"/wp-admin/admin-ajax.php";
			  	var request = new XMLHttpRequest();
				request.open('POST', ajax_url, true);
				request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
				request.onload = function () {
				    if (this.status >= 200 && this.status < 400) {
				    	alert("test");
				    }else {
				        // Response error
				    }
			  	}

			  	request.onerror = function() {
				        // Connection error
				};
				    
				request.send('action=cb_related_ajax');
				    
				};
		});
	</script>
	<?php
});

function cb_related_display_callback(){
	?>
	<button class="cb_relate_btn">Click Me</button>
	<?php
}

add_shortcode( 'cb_related_display', 'cb_related_display_callback' );