Issues with dynamic progress bar using Cast to HUD Widget Node

Hi - I am passing a sensor reading as a percentage value into the progress bar of a HUD Widget via my level blueprint.
I know the level blueprint may not be the best way to do this but I am relying on a plugin for reading serial data and need to stay with the level blueprint.
Can anyone explain what two nodes I need to connect to the ‘object’ part of the cast-to-HUD-Widget node in order to make this work?
I have seen in some screenshots that it is a Get Widget Variable. I’ve tried to create a widget reference variable (using every possible type of widget reference) but had no luck.