How to redirect output from a program that waits for input

Following this post I created my own version of the script, with the difference that user and password are forwarded from the environment variables:

#!/bin/zsh


pamtester login $user authenticate << eof 
echo $pwd
eof

How is it possible to redirect the output of the pamtester to a variable within the script? It looks like I always have the pamtester output to the terminal Password: pamtester: successfully authenticated or Password: pamtester: Authentication failure and everything that I insert after the eof line is ignored.