Loki basic auth towards mimir/prometheus alertmanager

[danieldagfinrud]

Context:
I’m running the latest available version of Loki in single binary mode using the helm chart here: loki/production/helm/loki at main · grafana/loki · GitHub and I am trying to send alerts to my Mimir Alertmanager (which is already in charge of my other prometheus alerts).

Question:
I have been looking through your documentation to see if I can find any way for loki to authenticate towards my Alertmanager with basic auth and I stumbled upon this:

Alertmanager authentication configuration for ruler

From what I gathered from this particular thread, it seems like this is possible but poorly documented. I’ve come up with the following configuration in my Loki helm chart:

rulerConfig: 
    wal:
      dir: /loki/ruler-wal
    alertmanager_url: https://mimir-alertmanager/alertmanager/
    alertmanager_client:
      basic_auth_username: ${loki_alertmanager_username}
      basic_auth_password: ${loki_alertmanager_password}
    ring:
      kvstore:
        store: inmemory
    enable_api: true
    rule_path: /loki/rules/fake/
    storage:
      type: azure
      local:
        directory: /loki/rules

The basic auth variables are mounted as extraEnvFrom for the single binary pods.
When I trigger some test alerts - I am seeing the following in my loki pods:

level=error ts=2023-05-16T15:04:25.585026679Z caller=notifier.go:534 user=1 alertmanager=https://mimir-alertmanager/alertmanager/api/v1/alerts count=1 msg="Error sending alert" err="bad response status 401 Unauthorized"

Any pointers in the right direction would be very much appreciated. If this ends up working, and there actually is no documentation for it, I will gladly contribute and add this as well.
Thanks!