Are you referring to setting the post content alignment to justified?
If you want to change it globally, you could add some custom CSS in your admin panel.
If you want to allow it as a direct formatting option when writing posts, Flarum supports the [ALIGN=justify] bbcode, but it's not enabled by default.
The only way to enable it is programmatically with the Formatter extender, something like this: (can be added to an extension or to your root extend.php array)
(new Extend\Formatter)
->configure(function ($config) {
$config->BBCodes->addFromRepository('ALIGN');
}),
