How to add a chat microservice inside a website

We are currently working on a RESTful website project using Java like ebay with posting, deleting, sorting, etc.. ads users put on the website.

We have a microservices architecture, each microservice responsible for a specific task (create an ad, a user service, etc.). Now we need to create a chat service.

The service should be able to make multiple users communicate between each other (multiple 1-to-1 chat rooms between users).

Which technology should we use ? How to link it with our user microservice ? Where to get started to implement this ?

Thanks for your clarification !