Custom email filter on text in email body

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From the Custom Email filter recipes in the Knowledge base, I see that you can filter on the From or Subject field, (e.g., if 822field from | grep -iq "sender@example.com"; then exit 99; fi)

I'd like to do the exact same filter but on a string in the body of the email. I tried, if 822field body | grep -iq "example.com"; then exit 99; fi

That doesn't seem to work. Any help on how to do this?