How to trigger codes to run each time a record mailmerge(MailMergeAfterRecordMerge)

I have some code that normally run on the document close event to register the document into a document management system. It works fine on a single document.

I now want to use it in a mail merge and run it for each record merge. The user will run the mail merge normally from the toolbar.

For each record that merges I want to pull some data from the mail merge data source and use it to register the document before merging the next record.

It seems that the MailMergeAfterRecordMerge is the event that I need to use but I don't quiet understand how to make it trigger.