Hi! To send alerts to an external Alertmanager go to the Alerting page, then click on the Admin tab. At the end of the page there is Send alerts to where its possible to change between Internal Alertmanager, External Alertmanager, or both.
By searching in the grafana code, I found this PR where it has been merged
It seems there is a alertmanagersChoice config that can be “external” but I don’t know how to make it work/where it should be located.