Where is the NET SEND in Windows 8.1?

What is the Net Send Command? 

(from http://pcsupport.about.com/od/commandlinereference/p/net-send-command.htm)

The net send command is a Command Prompt command used to send messages to users, computers, and messaging aliases on the network.
Windows XP was the last version of Windows to include the net send command. The Messenger Service in Windows XP needs to be started to send a net message  The msg command replaces the net send command in Windows 8, Windows 7 and Windows Vista.
The net send command is one of many net commands.

Net Send Command Syntax:

net send {name | * | /domain[:domainname] |/usersmessage [/help] [/?]
e,g, NET SEND AnAccountName "Hello, World"

What is the Msg Command?

(http://pcsupport.about.com/od/commandlinereference/p/msg-command.htm)
The msg command is a Command Prompt command that's used to send a message to one or more users on the network.
The msg command functions similarly to the net send command that was popular in Windows XP but it is not a true replacement for it. See Using the Msg Command to Replace Net Send further down the page.

Msg Command Syntax:

msg {username | sessionname | sessionid |@filename | *} [/server:servername] [/time:seconds] [/v] [/w] [message]
e.g.,: msg Charlie "Hello, World" or msg Charlie Hello World