Try this rule62:
1. Goto "MENU"
2. goto "MESSAGING"
3. goto "SETTINGS"
It should say
MSG SETTINGS
1.ALL MSG
2.TXT MSG
3.PICTURE-VIDEO MESSAGE
Now what you need to do is select "ALL MSG"
and somewhere in that list it should say,,
SIGNATURE
now go to signature, and select "CUSTOM"
and you should be able to put txt in that box for your signature.

Please PM me if i helped you
