How to include a system field in watcher

Quite new to Kibana and the whole ELK stack.
We have Kibana v7.2 currently, and we have a visual built to watch our memory usage on some servers.
I'm trying to use the watcher feature to setup alerts if the memory should go above 80%, but I cannot see an area where one would put the field I want to include:

system.memory.actual.used.pct

Like I said, I'm quite new to ELK, so I actually have no clue what to use right now. Trolling through the documentation is only getting me so far

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