Using Internal Lookup Field in Calculated Field

 Previously, when using a Typehead field we were able to reference a child field of the Typehead within a calculated field.

An example would be the Typehead field referencing a particular project, and then a calculated field being able to pull through the ID of that project using the notation: {DE:typeheadproject:ID}. 

This doesn’t seem to work for internal lookup fields which we have now been directed to use? Can you please let me know what the new notation is to reference these child fields from an internal lookup field?