How to put and post links with Spring HATEOAS
Look at the answer here:
POSTing a @OneToMany sub-resource association in Spring Data REST
Question 1) Yes this is how you post links/relations. With URIs.
Question 2) The URI of the resource actually IS its ID from the client's perspective. The server internally automatically resolves this URI into the actual model instance with
org.springframework.data.rest.core.UriToEntityConverter.convert(...)