You can add the following hooks in your code to add it to event-category custom taxonomy.
add_action('event-category_edit_form_fields','extra_category_fields');
add_action('event-category_add_form_fields','extra_category_fields');
You can add the following hooks in your code to add it to event-category custom taxonomy.
add_action('event-category_edit_form_fields','extra_category_fields');
add_action('event-category_add_form_fields','extra_category_fields');