Actually, that you cannot do. Try it and see.

Code:
 
alias xx {
  msg # test
  msg # $chr(32) test
}


Both lines echo test without leading spaces.

What you can do, is use $chr(160)

Code:
 
alias xx {
  msg # test
  msg # $chr(160) test
}


Where the second line has two leading spaces (1 for the hardspaced 160, one between the $chr(160) and the word test).