It is very easy, see:
Add a new sidebar on functions.php of your theme:
add_action( 'widgets_init', 'theme_slug_widgets_init' );
function theme_slug_widgets_init() {
register_sidebar( array(
'name' => __( 'Cart Widget', 'your_text_domain' ),
'id' => 'cart-header',
'description' => __( 'Your Description', 'your_text_domain' ),
) );
}
Now add code to get the sidebar on your header.php file:
<?php if ( is_active_sidebar( 'cart-header' ) ) { ?>
<?php dynamic_sidebar( 'cart-header' ); ?>
<?php } ?>
Go to WordPress in Widgets and add de Woocommerce Dropdown to Cart Widget Sidebar.