As @thiagoralves already said, the easiest way would be to use Modbus. Just have a look at node-red-contrib-modbus, there are even examples of how to convert the whole thing into Nodered.
A difficult and maybe also unsafe way would be to log into the Openplc webserver with an exec node
curl -c cookie.txt -d "username=openplc" -d "password=openplc" http://localhost:8080/login then read the corresponding data from http://localhost:8080/monitoring with
curl -b cookie.txt http://localhost:8080/monitoring (see also OpenPLC_v3/webserver/monitoring.py) and log out again with
curl -b cookie.txt http://localhost:8080/logout
Certainly possible, but quite cumbersome. Modbus is definitely simpler and more useful.