Wordpress Multisite User Admin Link

I would like to know how I can create a link for a multisite user that has a site installed as a subdomain so that when he clicks on it, it will take him to his admin panel. Right now I tried the link below but it redirects him to the wordpress error page that says "Visit Dashboard" / "Visit Site". I would like it not to redirect him to that but his admin dashboard.

href="/wp-admin/admin.php". (Doesn't work because he has a subdomain)

How can this be done?