Here you go, although this is for the Today screen left softkey. Save the ZIP to your PC or Phone, extract the CAB from it, and install the CAB. (Requires a Soft Reset after install.)
For the record, this CAB creates a shortcut in \Windows\AppButtons. The shortcut triggers the following command:
"tmail.exe" -service "SMS" -to ""
After it creates the shortcut, it sets the following registry key and value:
<characteristic type="HKCU\Software\Microsoft\Today\Keys\112">
<parm name="" value="New SMS" datatype="string" />
<parm name="Open" value="\Windows\AppButtons\NewSMS.lnk" datatype="string" />
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.
--Aaron