Hello @linh,
First off, welcome to the HiveMQ Community! Always happy to see new users joining in here in the community forum.
As for the SSL certificate, the server CA file can actually be obtained from right here in our community forum - here is a quick link to the thread, and here is a direct link to the server CA file.
This CA file can be used directly with some clients, or be used to create a client certificate if necessary following our how-to guides here. Note that you will only need to follow the client certificate creation steps listed here, as we already have the server CA file and HiveMQ Cloud brokers are already configured to support TLS.
Best,
Aaron from the HiveMQ Team