Could you clarify to me how to add the MergeWith switch so it will link the Opportunity to the hisotry created?

I implemented this code as an include script and I have it set to record a history but it isn't linking the Opportunity to the History.

On the Form Event itself this is all I have:

MailMerge EntityID, PluginID, 1, "True", "", "True", "True", "", "", "", ""

It performes the merge, creates the history, links the Account and the Contact to the history, but doesn't link the Opportunity. How do I go about having it link the Opportunity?

Thanks.