The validates requirement link is represented as a oslc_qm:validatesRequirement predicate and stored on test artifacts like a oslc_qm:TestCase as defined by the OSLC Quality Management specification. If you want links representing those semantics, they should use that predicate for compliance with that OSLC specification. You can find more details at https://docs.oasis-open-projects.org/oslc-op/qm/v2.1/ps01/quality-management-spec.html. Links between requirements and test artifacts should be stored on the test artifacts and not on the requirements. I don't think that ETM currently supports custom link types - feel free to submit an RFE against ETM.