How to create custom field in worpdress default widgets?

Simply edit your theme functions.php file,

find :
register_sidebar function

check out the name attribute in the function to make sure which widget you are going to update

There should be an attribute called before_widget

you can define your custom CSS Class name into it

here is the sample code for your reference

register_sidebar( array(
    'name'          => __( 'News Sidebar', 'left-test-sidebar' ),
    'id'            => 'sidebar-1',
    'description'   => __( 'Add widgets here to appear in your sidebar.', 'depnet' ),
    'before_widget' => '<section id="%1$s" class="widget %2$s YOUR-CUSTOM-CSS-CLASS">',
    'after_widget'  => '</section>',
    'before_title'  => '<div class="widget-title YOUR-CUSTOM-CSS-CLASS"><h4>',
    'after_title'   => '</h4></div>',
) );