net send [/domain:<workgroup/domain>] <computername> <text to send>
/domain:<workgroup/domain> is only applicable when you want to send messages to another workgroup/domain.
<computername> is the name of the computer where to you want to send it.
<text to send> can be uppercase/lowercase and/or with spaces.
Outside your domain/workgroup:
net send /domain:Another workstation4 Hello! Just a notice from me that your computer is h4x0r3d.
Within your own domain/workgroup:
net send workstation23 Hello! Just a notice from me.