How to add elements in a many-to-many relationship via Spring's @RepositoryRestResource REST API?

I found an answer here: How to update reference object in Spring-data rest?

By using "Content-Type: text/uri-list" instead of JSON, it is possible to "add" a resource to the collection with a PUT and pass in the URI. You can remove the resource with a DELETE.

After some digging, I discovered that the Spring documentation does describe this: http://docs.spring.io/spring-data/rest/docs/2.2.0.RELEASE/reference/html/#repository-resources.association-resource.

I always hated that text/uri-list content-type, so I did some research and it turned out there is also an undocumented JSON format which can be used:

{
  "_links":{
    "rel":"/555",
    "rel":"/556"
  }
}

The rel of the links could be anything except empty string, they could be all the same. The link part could be the whole URL form the self link of the referenced object, but the last part of the URL is enough. ( forseslash +id)