To call a script xml-rpc method, you need to use the TriggerModeScriptEventArray basic xml-rpc method. The script xml-rpc methods always take an array of string as parameter.
query('TriggerModeScriptEventArray', ['NameOfTheScriptMethod', ['Parameters', 'of', 'the', 'script', 'method']])
Be aware that when you call a script xml-rpc method, the answer you get is the answer to the 'TriggerModeScriptEventArray' basic xml-rpc method and not the one from the script method. Script methods that must return an answer will send it using a script callback.
Like the basic xml-rpc api, the script xml-rpc callbacks are disabled by default and you need to turn them on manually if you want to use them.
query('TriggerModeScriptEventArray', ['XmlRpc.EnableCallbacks', ['true']])
Once the script callbacks are enabled you can listen for the ManiaPlanet.ModeScriptCallbackArray basic callback that will return two values. A string that is the name of the script callback and an array of strings that are the values of the callback.
The script xml-rpc api uses a different versionning system than the basic xml-rpc api. You can get all the available version by using the XmlRpc.GetAllApiVersions script method and XmlRpc.AllApiVersions script callback. Then set the version you want to use with the XmlRpc.SetApiVersion script method. By default, the latest version is used.
You can find the script xml-rpc api documentation on this GitHub repository.