add_action( 'wp_head', function () { ?>
<script>
jQuery(document).ready(function($) {	
$('#wpforo #wpforo-wrap .wpforo-admin-note').append($('<div class="usercount"> На форуме зарегистрированы <?php $stat = WPF()->statistic(); ?> <?php echo $stat; ?> пользователей. Стань <?php echo $stat+1; ?>!</div>'));
});
</script>
<?php } );

I don't know where the problem is on your side
for me the code works without any problem

use the plugin if it doesn't work for you via functions.php

you asked for a way to add php to the admin note

the php code above is just an example, you can use your own

in the page code - there is a script. but no text is displayed on the page. and further. in my case I need: "153 users are registered. Become 154" (not 153+1)

and after the main text, not before

yes 😀 😉 it works

https://www.screencast.com/t/qXBRjsZFW2qV

you can use this code

add_action( 'wp_head', function () { ?>
<script>
$(document).ready(function() {	
$('#wpforo #wpforo-wrap .wpforo-admin-note').prepend($('<div class="usercount"> <?php $stat = WPF()->statistic(); ?> <?php echo $stat; ?> users are registered. Become <?php echo $stat; ?> + 1</div>'));
});
</script>
<?php } );

either via the Code Snippets plugin
or in the functions.php of your WP theme

you also have to add text to the admin note, to make it active