Add-on =/= integration.
An add-on is some additional software service such as Adguard or Deconz or an MQTT broker.
An integration is used to integrate a device/service into Home Assistant.
To use the Volvo On Call integration, follow the docs you linked, they explain exactly how to enable it. This one can’t be done through the UI, you need to add config to configuration.yaml
I’m not entirely sure the Volvo integration even works anymore. And I mean, like it might have become deprecated in the past several days and whoever maintains the integration doesn’t even realize it yet.
I used it a few months back and it worked fine, I just didn’t need it at the time, so I removed it. I just tried to re-install it and nothing happens.
Meanwhile, this recent post points out that there’s suddenly a new Volvo dev portal. I think perhaps Volvo has shut the legacy API down and you can no longer connect with basic auth.
Ok, it cleared a few things up, but the actual integration with mqtt etc, I have at this point no clue about, but I guess the answer can be found if I search a bit.