You should be able to just add the field to the create event mapper and specify the relevant value.

Sorry I meant is there a way to attach the source email to the event as a .eml file?

Same goes if it’s an update - I’d like the source .eml file to be attached to the action.

I think you’ll just need to select the Channel option “Expose Original Message as Header”. The iteration expressions for the attachment mappers will then add the eml as well as any attachments in the mail.