Or just go straight to the following registry entry:
<characteristic type="HKCU\ControlPanel\Sip">
<parm name="DefaultIm" datatype="string" value="{BB25F2EB-688C-4698-ADFC-F422A148AD4F}"/>
</characteristic>
That CLSID is for PCM Keyboard. You'd have to enter the CLSID for the HTC Keyboard you want to use. For the Phone Keypad its: "{198D503A-EFFE-4afd-BC54-EED333C80E6D}"