Updating a related record dropdown when meeting created


Can anyone point me in the right direction of how I can update a dropdown in Leads when a Meeting is created via the sub panel in Leads.  I am trying to write a logic hook to do it but can't see how to access Leads from Meeting using before_save or after_relationship_add.

I have searched and searched and can't see anything that helps!

Using 10.2.0 cloud