NewSMS_Softkey.cab sets the LEFT softkey on the Today screen as "New SMS"
and allows you to start a new SMS message from the Today screen.

This CAB creates a shortcut in \Windows\AppButtons that triggers the
following command:

"tmail.exe" -service "SMS" -to ""

After it creates the shortcut, it sets the following registry key and value:

[HKCU\Software\Microsoft\Today\Keys\112]
@="New SMS"
"Open"="\Windows\AppButtons\NewSMS.lnk"

As long as you are using a stock ROM, it will work fine.

If not, you may have to fix your shortcut to launch the correct service...
ie. "MMS" or whatever. If you want it to launch a new Hotmail, for example,
change the service to "Hotmail". If you need to identify the service name,
browse to HKLM\Software\Microsoft\Inbox\SVC, and then just fix the shortcut
in AppButtons using notepad.