Where to put Uniform Resource Names (URN)?

Uniform Resource Names (URN) are used to uniquely identify publications. For instance some national libraries define URNs in the urn:nbn namespace. Unique. persistent identifiers are cruicial to refer to publications without duplicated, so it would be very helpful to include them in references. There is a field for DOI but where to put URN? Could you please add a field for URNs?

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