Identify the recipient in side conversation

I would need to report on side conversations.

The aim is to have the number of side conversations sent to a well-defined email address.

I tried to creat a trigger to add a tag to tickets every time a side conversation is created and sent to this address. The probleme is that I don't have any Action on triggers that allow me to choose the recipiant of the side conversation. Particularly as the recipient address of the side conversation is different from the requestor's address and the recipient is an end user in Zendesk.