Passing Newline character as an argument within shell

Easiest but not very pretty:

./myprog "
"

EDIT: Still easiest, but I forgot about it:

./myprog $'\n'

Another alternative:

./myprog ^M                  # not ^ and M, but the literal LF character
                             # in `bash`, obtained by Ctrl-V, Enter

Or, you can do what these programs do, and parse the argument yourself. If you say ./myprog "\n", your code will receive the two-character string \n as its first argument - look for such a sequence and translate it to a newline (e.g. by passing the argument through printf or equivalent in your language, or by regular expression substitution...).