I don't know why you want to restart a UF using REST API, I think that the easiest way is a remote shell script.
Anyway you can find all the information about REST API features at http://dev.splunk.com/restapi .
If you want to run above command from remote server then you need to change default password for admin user otherwise you will get below error.
<?xml version="1.0" encoding="UTF-8"?>
<msg type="WARN">Remote login has been disabled for 'admin' with the default password. Either set the password, or override by changing the 'allowRemoteLogin' setting in your server.conf file.</msg>