New Post: ProgressText of custom action

I'm using the custom action to run the installed app at the end:

new ManagedAction("LaunchServer", Return.check, When.After, Step.InstallFinalize, Condition.NOT_Installed)

    public class CustomActions
    {
        [CustomAction]
        public static ActionResult LaunchServer(Session session)
        {
            Process.Start(session["SERVERDIR"] + @"\Server.exe");
            return ActionResult.Success;
        }
    }

I wonder how to set up the text which is shown in the progress bar when this custom action is being executed, in Wix it normally achieved by:

<UI>
  <ProgressText Action="LaunchServer">Running Server... (this may take a few minutes).</ProgressText>
</UI>

However, I can't find any property/method in WixSharp that allows to do that.
Any help is greatly appreciated!