Add a URL parameter to the shared URL

You can do that in JavaScript using AddToAny events. See “Modifying the share”:

https://www.addtoany.com/buttons/customize/wordpress/events

@micropat How can I pass a PHP data onto it, e.g. a specific user meta field?

How can I pass a PHP data onto it, e.g. a specific user meta field?

Any update on this?

add_action( 'wp_head', function() {
	$the_referrer_code = '';
	echo '<script>';
	echo 'var the_referrer_code = "' . $the_referrer_code . '";';
	echo '</script>';
}, 9 );

Replace the value for $the_referrer_code as needed.

Then you can reference the_referrer_code in your Additional JavaScript box in AddToAny settings.