Adding Code into Theme Customizer Header

You use the customize_controls_enqueue_scripts action hook for that. Then you enqueue script as usual. Like this:

function wpse256911_customizer_enqueue() {
    wp_enqueue_script( 'my-customizer', get_stylesheet_directory_uri() . '/js/my-customizer.js', array( 'jquery', 'customize-controls' ) );
}
add_action( 'customize_controls_enqueue_scripts', 'wpse256911_customizer_enqueue' );