theming – hook_form_FORM_ID_alter: Pre select a checkbox from an exposed filter in a drupal 8 view

I have a view that lists blog articles. The blog content type has a taxonomy reference field to the ‘tags’ vocabulary, authors can select 1 or multiple tags. The view exposes the ‘Has taxonomy terms (with depth) (exposed)’ filter (as a list of checkboxes) so that users can search for blog articles containing 1 or more tags.

Now, i’m trying to pre-select 1 of the checkboxes that are exposed to the user in the hook_form_FORM_ID_alter() hook. It should be a simple as the code below but it just doesn’t work. The tag i’m trying to pre-select has the ID 288.

What am i doing wrong? Thx…

function xtheme_form_views_exposed_form_alter(&$form, DrupalCoreFormFormStateInterface $form_state, $form_id)
{

    if($form('#id') == 'views-exposed-form-vcon-finder-page-1'){
        $form('tags')('#default_value') = (288 => 288);
    }

}