See https://docs.jboss.org/author/display/TEIID/System+Procedures, sysadmin.logmsg method. You can define your own log context or use one already defined. Log Context here is same as log4j log contexts that are defined in the main configuration file.
I've just put diffrent calls to this procedure and when I save, the transformation editor show the message: ERROR: SYSADMIN.logMsg does not exist.
Check the version of designer. 7.6 has an out of date definition of the system schema. 7.7 should be updated.
I've installed Teiid designer (v. 7.8) and teiid server (Versión 7.7) but the problem is still the same.
You'll want to log a JIRA against Designer. If you are using the latest, that procedure should be available at design time. You can also verify that against the server you can directly execute "call logMsg(level=>'ERROR', msg=>'an error')" and see that you get an error log.