Any problem with just a password? This is Exim 3.x:
######################################################################\n# AUTHENTICATION CONFIGURATION #\n######################################################################\n\nlogin:\n driver = plaintext\n public_name = LOGIN\n server_prompts = "Username:: : Password::"\n server_condition = "${if eq{$2}{mysecretpassword}{yes}{no}}"\n server_set_id = $1\n\n~