Actually, that you cannot do. Try it and see.
alias xx {
msg # test
msg # $chr(32) test
}
Both lines echo test without leading spaces.
What you can do, is use $chr(160)
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).