At first you have to use one of authorization methods (they are described in the API documentation). For example, by using "authinfo":
Code:
https://your_ip/manager/ispmgr?authinfo=admin_name:admin_password&out=xml&func=services.restart&elid=HTTP
The other way is to use "/usr/local/ispmgr/sbin/mgrctl services.restart elid=HTTP" (if you have to use the script locally).