unfortunatelly there is no process guid attribute in the Post/Sent Event. But what if you put the process guid in one of the parameters and map this parameter through the Event Manager GUI to the process guid?
Thanks for confirming what I thought was the case, as in there isn't a Process GUID in the Post event.
I didn't try this, as in the documentation it says you should avoid mapping to a GUID. But I will give this a test, in our test system, now and let you know how it goes. However, wouldn't mapping a parameter value to a GUID overwrite the GUID?
Another thought I had, was to use a calculation on the Events business object, to copy from a parameter field, into the ev_process_guid attribute, if the title matches this particular event?
The postmessage.aspx always creates a new process, it will not update an existing one. The only way you can get the behaviour you are requesting is to setup Event Manager to log new tickets in an unused domain such as Call, and then in the Call process, make a web service call to Event Manager that updates Incident or whatever process you are trying to update.