How to enable “Custom fields” option for custom post type

You must edit your custom post type code to add the custom fields support. Here is the example code you may follow:

$args = array(
        'labels'             => $labels,
        'public'             => true,
        'publicly_queryable' => true,
        'show_ui'            => true,
        'show_in_menu'       => true,
        'query_var'          => true,
        'rewrite'            => array( 'slug' => 'quiz' ),
        'capability_type'    => 'post',
        'has_archive'        => true,
        'hierarchical'       => false,
        'menu_position'      => null,
        'supports'           => array( 'title', 'editor', 'thumbnail', 'custom-fields' )
    );

Thank you both, I thought there maybe some easy option like enabling some checkbox or something else from front end. For now I am not going with installing plugin or adding any additional code.

Was under the impression that option to create custom fields should be available to all post types by default.

I thought to go ahead with plugin and created a custom field of type Wysiwyg Editor but the problem is that content is not formatted properly. It is showing all of them in a single lines as shown in the image below.

Any idea what needs to be done here.