Configuring Event logger with 'user-defined custom fields' in IWS V8.1

Hello Experts.

Can anybody help me with configuring "user-defined custom fields" in Event logger.

We can send 'comments' using built-in 'SendEvent' function.

But how to send data for custom fields. I can see 'Ev_Custom 1' column in SQL table.

In help section I could see "Format of the event history" is showing field number P14 to P23 is for user-defined custom fields.
Can any body guide me for this ?