Simple echo not working on Admin Area using hook

[theotoke]

Hello! I'm trying to make a simple hook to echo something, but it's not working on the admin area.

My WHMCS version is 8.2.1

This is my code:

<?php 

use WHMCS\Session;
use WHMCS\Ticket\Watchers;

function testing($vars) {
	echo 'test';
}

add_hook("AdminAreaViewTicketPage",1,"testing");

It works on normal client pages like ClientAreaPageProductDetails etc, but not on admin area ones.

Am i missing something?