DB architecture: What if a table is part of another table
Two ways to do it:


Option 1, as you suggest, is to make a new LeadId FK column in the [Reference] table, and make it nullable. This will work, it'll be easy enough to query, but you'll have a column with lots of nulls.
Option 2 is the 3NF (third normal form) way, with a third table that links [Reference] to [Lead]. That would contain a ReferenceId FK and a LeadId FK. You would need to join to this table in queries, but you wouldn't be stuck with a lot of nulls.