function myplugin_activate() {
   
    $upload = wp_upload_dir();
    $upload_dir = $upload['basedir'];
    $upload_dir = $upload_dir . '/mypluginfiles';
    if (! is_dir($upload_dir)) {
       mkdir( $upload_dir, 0700 );
    }
}
 
register_activation_hook( __FILE__, 'myplugin_activate' );