Linked Form Not Allowing New Records

I have a one to many relationship between two tables: tblCars and
tblParts (One car can have many parts). I created a form using the
wizard and chose "linked forms" instead of "form with subform" when
prompted. The problem Im having is that when entering new records in
the linked form it does not capure the "linked" field from the main
form which causes an error message which reads "Index or primary key
cannot contain a null value". I know access is supposed to "autofill"
that field but its not working. Whats more frustrating is that it
works fine for existing records. I only have a problem during data
entry. Any ideas?

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