Passing Data to a Service - Templating Question?

Happy weekend all!
Im after a little pointer please.

I have a notify service;

  - name: ga_broadcast_device
    platform: rest
    resource: !secret assistant_relay_url
    method: POST_JSON
    message_param_name: command
    data:
      user: "Home Assistant"
      broadcast: "true"
      talkback: "false"
      device : "Office speaker"

As well as sending “command” to this service, how can I dynamically pass the “device” to use too (so its not hard coded as “Office speaker”?

I assume something like calling the service with

message: test
data:
  device: "Office speaker"

But then what changes are required to my service to receive this data?
Thank you