Create a Link Field
When you create a custom object, you must provide a unique link field. A common unique ID is an email address, but for cars, the VIN (vehicle identification number) is perfect.
In the Fields tab, click New Field.
Name the link field, add an optional description, and be sure to select the Link data type.
Select whether the custom object is for a lead (person) or a company.
Create LInk FIelds