For your reference you can check following command reference which has some information about the command you are using:
http://www.cisco.com/c/en/us/td/docs/ios/12_2/security/command/reference/fsecur_r/srfathen.html#wp1018209
Extracted from this link:
Some protocols (for example, TACACS+) have the ability to override the use of local username prompt information. Using the aaa authentication username-prompt command will not change the username prompt text in these instances.
The aaa authentication username-prompt command does not change any dialog that is supplied by a remote TACACS+ server.
To change the prompt to the one you need you will have to do it from the Tacacs server side.
my question is where in the CPPM do i change this?