Translatable texts in module overrides

[Active]

I added an override for module ps_linklist: /override/modules/ps_linklist/ps_linklist.php
In that file I have a simple code: 

class Ps_LinklistOverride extends Ps_Linklist
{
    public function renderWidget($hookName, array $configuration)
    {
        $widget = $this->trans('Custom text', array(), 'Modules.Linklist.Shop');
        $widget .= parent::renderWidget($hookName, $configuration);
        return $widget;
    }
}

As you can see, that code displays 'Custom text' before original widget code. 'Custom text' is displayed in Front Office as expected. So override works correctly. But I couldn't find a way to translate 'Custom text'. It is simply not available in the list of translatable strings in Translations > Modules > Link List

Does anybody know how to translate that text?