Add Field::flagAutojoin to some fields only for specific template/pages

[Zeka]

Hi.
For now I add flag at init.php and remove it in ready.php like

// init.php
if (strpos(sanitizer()->entities(input()->url()), 'some/page/')) {
	fields('somefield')->addFlag(Field::flagAutojoin);
}

// ready.php
fields('somefield')->removeFlag(Field::flagAutojoin);

But there are pages that not so easy to identify by URL.

So far I couldn't find any suitable hook and probably it is the only way to do it, or I missed something? 

Thank!