How to set custom enum attributes using extended properties?

Hello,

Im trying to set the custom enum value of a DNG artifact using extended properties of a requirement object. But im getting a 403 forbidden error. Im able to set the string attributes using the same code but not enum attributes. Any hints on this?

QName targetQName = new QName(namespaceURI, localPart, prefix);
Map<QName, Object> reqMap = requirement.getExtendedProperties();
reqMap.put(targetQName, “rdf uri of enum value”);
requirement.setExtendedProperties(reqMap);