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requirement.getExtendedProperties().put(new QName("http://www.ibm.com/xmlns/rdm/types/Link"), new URI("http://google.com/resource"));

Link link = new Link(URI.create("http://google.com/resource"), "link To");
requirement.getExtendedProperties().put(new QName("http://www.ibm.com/xmlns/rdm/types/Link"), link);

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I have also tried using QName with 
    new QName("http://www.ibm.com/xmlns/rdm/types/", "Link");