CPI MonitoringxSLT eXTENDED FUNCTION TO SET CUSTOM HEADER PROPERTIES?

Dear experts,

I know how to set properties with extension functions in an XSLT Mapping.

I am also aware of how custom header properties can be set in a groovy script:

if(IDOCNUM!=null)

messageLog.addCustomHeaderProperty("IDOCNUM", IDOCNUM).

How do I achieve the same in an XSLT Mapping: Setting a custom header property in an XSLT Mapping so it becomes visible in CPI-Monitoring and can be searched for?

Many thanks!