code: public String link(ParaObject obj, String id2) {
  		if (obj == null || obj.getId() == null || id2 == null) {
  			return null;
  		}
  		String url = Utils.formatMessage("{0}/links/{1}", obj.getObjectURI(), id2);
  		return getEntity(invokePost(url, null), String.class);
  	}
docs: Links an object to this one in a many-to-many relationship.
  Only a link is created. Objects are left untouched.
  The type of the second object is automatically determined on read.
  @param id2 link to the object with this id
  @param obj the object to execute this method on
  @return the id of the {@link com.erudika.para.core.Linker} object that is created