The value itself isn't stored in a macro directly, you would need to either parse it from the $SERVICEOUTPUT$ macro:
https://assets.nagios.com/downloads/nag ... viceoutput
OR from the $SERVICEPERDATA$ macro:
https://assets.nagios.com/downloads/nag ... ceperfdata
Both of those should contain it (usually, it depends on the plugin you're using).
For User Macros you can use this:
https://assets.nagios.com/downloads/nag ... ponent.pdf
Or manual management through the resource file:
https://assets.nagios.com/downloads/nag ... Macros.pdf
Or through Custom Object Variables:
https://assets.nagios.com/downloads/nag ... tvars.html
EDIT: You can also submit a feature-request here for Nagios Core if you'd like to request a new macro as well:
https://github.com/NagiosEnterprises/nagioscore/issues