This is the unique key of the Custom Field you wish to modify.

This is the Id of the subscription product in which the custom field exists.

This is the value you wish to update the custom field to. This will always be the string version of the value. If you wish to pass a number, for example, pass it as "123". Likewise, a DateTime would be passed as "2014-06-26T04:00:00". We will use the known data type of the custom field to properly convert the String to a Number, DateTime, etc.