how to add custom css at top above all css file for specific url

/**
 * Enqueue stylesheet before others
 */    
class EnqueueStylesheetEarly {

    function __construct() {
        add_action( 'wp_enqueue_scripts', [ $this, 'load_before_other_stylesheets' ], 5 );
    }

    /**
     * Register and enqueue stylesheet
     */
    function load_before_other_stylesheets() {
        wp_register_style(
            'bootstrap-css',
            plugin_dir_url( __FILE__ ) . 'bootstrap.min.css',
            [],
            '3.3.7'
        );

        wp_enqueue_style( 'bootstrap-css' );
    }

}

Then you can load this with your other frontend classes in your ‘plugin.php’ Like so…

if ( ! is_admin() ) {
    require 'class-enqueue-stylesheet-early.php';
}

At a 5 priority, this should solve your problem. The only thing that usually loads before this is WordPress SEO JSON-LD scripts and meta.