Source

File: wp-includes/link-template.php

function get_admin_url( $blog_id = null, $path = '', $scheme = 'admin' ) {
	$url = get_site_url( $blog_id, 'wp-admin/', $scheme );

	if ( $path && is_string( $path ) ) {
		$url .= ltrim( $path, '/' );
	}

	/**
	 * Filters the admin area URL.
	 *
	 * @since 2.8.0
	 *
	 * @param string   $url     The complete admin area URL including scheme and path.
	 * @param string   $path    Path relative to the admin area URL. Blank string if no path is specified.
	 * @param int|null $blog_id Site ID, or null for the current site.
	 */
	return apply_filters( 'admin_url', $url, $path, $blog_id );
}