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Go to Profile > A Non-default Project Mapping > Keyword Rules and press New Keyword Mapping.
This will then open the configuration for the New Keyword Mapping. Here you enter the relevant keyword/s and select whether you would like to also search within the Email Body. Once configured press Save.
Now you will be able to configure the other tabs of the Keyword Rule. e.g. Issue, Email and Custom Field Defaults.
Once everything is configured and saved, you will be able to view the Keyword Rule config within the Keyword Rule Summary.
Configuration options
Include Body in Keyword Scan
This is will include the Body within the Keyword Scan. If disabled then only the Subject will be checked.
Keywords (CSV)
Comma separated list of Keywords that will be checked for within the email. Keywords can be regular expressions (avoid using .* in that case).