It depends what you want to cast, but just as a 'mockup' example, here I am controlling Chrome with the Google Cast extension installed.
<?xml version="1.0" encoding="utf-16"?>
<!--VoxCommando 2.2.2.7-->
<commandGroup open="True" name="Chrome control" enabled="True" prefix="" priority="0" requiredProcess="" description="">
<command id="83" name="Chromecast song {1}" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="Once the modal popup opens, this point you'll need to add mouse emulation (see the mouse.*** actions) to select a chromecast device. Not sure how reliably it will work, since I can't test it. You'll need to experiment with adding VC.Pause in between emulations as needed, probably.">
<action>
<cmdType>Tools.Encode.URI</cmdType>
<params>
<param>{1}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>Launch.RawParam</cmdType>
<params>
<param>{Path.PFx86}\Google\Chrome\Application\chrome.exe</param>
<param>C:/Users/Public/Music/Sample%20Music/{LastResult}.mp3</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>VC.Pause</cmdType>
<params>
<param>1500</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>Window.Maximize</cmdType>
<params />
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>Window.Focus</cmdType>
<params>
<param>chrome</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>Mouse.MoveAbsolute</cmdType>
<params>
<param>500,500</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>Mouse.RightClick</cmdType>
<params />
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>VC.Pause</cmdType>
<params>
<param>50</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>InputKeys.Send</cmdType>
<params>
<param>{c}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>Chromecast song</phrase>
<payloadList>Kalimba,Maid with the Flaxen Hair,Sleep Away</payloadList>
</command>
<command id="76" name="Close Chrome" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="">
<action>
<cmdType>Window.Close</cmdType>
<params>
<param>chrome</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<phrase>Close Chrome</phrase>
</command>
</commandGroup>
The example uses some default songs installed on everyone's Windows machines, usually. So it can be used as a test case as is. But I don't have a Chromecast so the command does not include the mouse emulation needed to select a device.