No, because the default value will be needed before DealerID is known (eg on INSERT)
Edit:
This means that SQL Server does not the value in the table at the time of insert, only after. Therefore, it can not a UDF for the default.
For example, what about a multiple row insert, or where you have NEWID() default?
Now, using logic basic on DealerID: if it's GUID, why? It's an internal, non-user readable value.
If you really need this, you'll have to use a computed column for the "base" value and another column for the "actual" value with ISNULL.
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