I was unclear - I've added this variable myself.
I must say I didn't knew the first dialog box you shown, "Edit system variable"
So what you done via the Run dialog box is the same as to execute your script (c:\scripts\speak.vbs) with "this is a test" as parameter.
Which is pretty much the way I'm already doing it. What I would like to know is if there's any way to use a custom system variable something like this;
So to run your script you should write something like that :VBSScript := "c:\scripts\speak.vbs" TextToSpeak := "this is a test" RunWait, cscript.exe //nologo "%VBSScript%" %TextToSpeak%",,Hide
ExecuteSystemVariable, TTS "%name%"