I can't decipher what you are asking exactly...
If the client binds a key to "messagemode test" and pushes the key, they will see a prompt in the upper-left hand corner that says "test:". They can type whatever message they want, push enter, and it will display only to them: "You typed: "blahblahblahblah"".
If you want the client to simply bind a key to amx_test, and see the "test:" prompt in the upper-left when the key is pushed, add the following to your existing code.
Code:
register_clcmd("amx_test", "test_function")
public test_function(id)
{
client_cmd(id, "messagemode test")
}