writing to event log in a custom process

I created a custom class for a process that does some basic manipulation to an HL7 message. I'd like to be able to write some debugging to the process event log. How would I do that?

I've tried ##class(Ens.Util.Trace).WriteTrace("system","HOSP.IHE.PDQProcess","MessageHeaderHandler","Discarding input message")

but I get a runtime error when I try that. I know this is pretty basic stuff, but any help would be appreciated.