Error: cannot assign values to constants

Regarding to pass a parameter/argument to my compiled .au3 ->.exe, I read the help on Command Line Parameters zillions times, but I still don't get it right.

my sample code is: "sample.au3"

$CmdLine[0] = 1
MsgBox(0, "", "The argument is " &  $CmdLine[1])

I compile to sample.exe. When I run it in DOS command line:

sample.exe "123"

I always have Autoit error message popup:

AutoIt error:

Error: Cannot assign values to constants

I've been search out some similar questions in this forum, some answer like:  $CmdLine[1] is a constant, can not be used as by passing in a variable.

But hey, that is my purpose. I need to pass a variable into a AutoIT compiled .exe. I need to call autoit compiled .exe and passing in a variable in my C# code. However I failed to do it even in DOS command line.

Again, I already read the HELP regarding "Command Line Parameters", I already search the similar topic in this forum, please help to show me how to correct my sample code make it work.