// Generate a new Announcement  Popup Content WYSIWYG editor field for the announcements
function custom_popup_content_field( $fields ) {
    $fields[] = [
        'name'    => 'Announcement Popup Content',
        'desc'    => 'Enter the popup content for this announcement.',
        'id'      => 'announcement_popup_content',
        'type'    => 'wysiwyg',
    ];

    return $fields;
}
add_filter( 'timeline_express_custom_fields', 'custom_popup_content_field' );