Hi Mark,
there is a new function introduced within 11.2, if you got that version. It's called "Prognosis Alerts/CI" but please read the documentation to check if that helps with your requirement. If you're using standard email alerts or SNMP traps I don't think you can add that custom defined filed. You would need to write the IP hardcoded.
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The Prognosis Alerts/CI function is used to add Configuration Items (CI) and extra secondary data to alert messages generated by Prognosis. It does this by mapping the server name contained in the alert message to the same server name as defined in the CI Mapping database. This allows alert messages to be enhanced with additional information so that they can be used in third party configuration management database (CMDB) and ticketing systems, such as Service Now, within the ITIL framework.
Find more under Home > System Functions > Thresholds > Alert/CI Integration
