通过API在loopback.io中创建具有直通关系的实例的正确方法是什么？

I've two models, A and B which extends user buildt-in model. They both have a relation "has many" with each other using a through model A_B. I want to add an instance of B related with an existing A instance. I've try to post the new B instance with a field As ("As" is the relation name) setted with an array containing the A instanceI've two models, A and B which extends user bui