Customize wp-admin form custom fields

I am not sure this is your problem if you are not getting any errors, but make sure you have your jQuery code within document ready wrappers and pass the $ in to make sure you can access it since WordPress runs in no-conflict mode.

jQuery(document).ready(function($) {

    $("input").keyup(function(){
        $("input").css("background-color","pink");
    });

});

It would also be good to declare jQuery as a dependency just to be safe.

wp_enqueue_script( 'my_custom_script', get_template_directory_uri() . '/js/admin.js', array( 'jquery' ) );