Google around, I found the instruction here.
So we need to create 2 sites: clientdomain.com and www.clientdomain.com where one site (www.clientdomain.com) is configured to redirect request to the other (clientdomain.com) by using the HTTP Redirect module.
To bind the host clientdomain.com, we add site binding with Type: https, IP address: All Unassigned, Port: 443, Host name: clientdomain.com, SSL certificate: (select the SSL certificate)
To bind the host www.clientdomain.com, we need to use the command appcmd:
appcmd set site /site.name:"www.clientdomain.com" /+bindings.[protocol='https',bindingInformation='*:443:www.clientdomain.com']