Resolution:
In the workflow, use UpdateEvent activity and in that activity pass eventDescriptor as in parameter with a value. The value can be got from the domainentry table where domaintype is APPFUNCTION using the following SQL:

select * from domainentry where domaintype like 'APPFUNCTION' for validvalue list.

This is the same as subtype attribute in MLXML document. It only has to be set if workflow is initiated by document of unknown type. Also one  can create new entries in this table and use the value from table as value to the eventDescriptor  parameter.