WFFM format success message in custom Save Action.

gistfile1.cs

public class FormatSuccessMessage
{
    public void Process(SubmitSuccessArgs args)
    {
        var script = FormSaveActionUtilities.GetGoogleAnalyticsScriptFromSaveAction(args);

        Assert.IsNotNull((object)args, "args");
        if (args.Form == null)
            return;
        args.Result = args.Form.SuccessMessage + script;
    }
}