you mean something like :
[root@tigerente ~]# echo "TEST^D" | write chakkerz
write: chakkerz is logged in more than once; writing to pts/7
and on pts/7:
Message from firstname.lastname@example.org
on pts/6 at 16:09 ...
the CTRL+C you mention i assume you want to use to terminate the message, if that is correct, you really want to use CTRL+D which is an EOF character. To put that in your message you use CTRL+V then CTRL+D so that TEST^D is actually TEST^V^D
hope that helps