define variable in alertmanager configuration yaml file to forward prometheus alerts to slack

Question

I am stuck with a basic YAML problem but I am not sure what's the proper way around it and YAML isn't my strong suit.

I am trying to get the below code to work. The "slack_configs:" section is where I get an error. Whatever I put under the "templates.tmpl" section does not seem to be picked up.

Here's the code:

alertmanager:
    baseURL: https://alertmanager.acme.com/
    ingress:
        enabled: "true"
        ingressClassName: nginx
        annotations:
            kubernetes.io/tls-acme: "true"
        hosts:
            - alertmanager.acme.com
        tls:
            - secretName: alertmanager.acme.com
              hosts:
                - alertmanager.acme.com
## alertmanager ConfigMap entries
##
alertmanagerFiles:
    alertmanager.yml:
        global:
            slack_api_url: https://hooks.slack.com/services/ACME
            opsgenie_api_key: 1234
        route:
            group_wait: 10s
            group_interval: 1m
            repeat_interval: 2h
            receiver: slack
            routes:
                - receiver: slack
                  repeat_interval: 1m
                  continue: true
                  continue: true
                - receiver: OpsGenie
                  repeat_interval: 1m
        receivers:
            - name: slack
              slack_configs:
                - channel: '#alertmanager'
                  send_resolved: true
                  title: '[{{ .Status | toUpper }}{{ if eq .Status "firing" }}:{{ .Alerts.Firing | len }}{{ end }}] {{ .GroupLabels.SortedPairs.Values | join " " }} {{ if gt (len .CommonLabels) (len .GroupLabels) }}{{ with .CommonLabels.Remove .GroupLabels.Names }}{{ .alertname }}{{ end }}{{ end }}'
                  text: |-
                    *Cluster:* {{ template "clusterName" . }}
                    {{ range .Alerts }}•  (<{{ with .Labels }}{{template "alertmanagerURL" . }}#/silences/new?filter=%7B{{ range $key, $value := . }}{{ $key }}%3D%22{{ $value }}%22%2C%20{{end}}alertname%3D%22{{end}}{{ .Labels.alertname }}%22%7D|Silence>) (<{{ .GeneratorURL }}|Source>) {{ .Annotations.description }}
                    {{ end }}
            - name: OpsGenie
              opsgenie_configs:
                - responders:
                    - name: Oncall
                      type: team
    templates.tmpl: |-
        {{ define "alertmanagerURL" }}https://alertmanager.acme.com/{{ end }}
        {{ define "clusterName" }}acme-eu-west-1{{ end }}
server:
    global:
        external_labels:
            cluster: acme
    baseURL: https://prometheus.acme.com/
    ingress:
        enabled: true
        ingressClassName: nginx
        annotations:
            kubernetes.io/tls-acme: "true"
        hosts:
            - prometheus.acme.com
        tls:
            - secretName: prometheus.acme.com
              hosts:
                - prometheus.acme.com

The prometheus alerts are not forwarded to slack. When I check the logs in the alertmanager pod, I get the following error message:

level=error ts=2022-05-23T07:16:16.547Z caller=dispatch.go:354 component=dispatcher msg="Notify for alerts failed" num_alerts=5324 err="slack/slack[0]: notify retry canceled due to unrecoverable error after 1 attempts: template: :1:23: executing \"\" at <{{template \"clusterName\" .}}>: template \"clusterName\" not defined"

The error is pretty clear and self explicit. It can't find the variable I am trying to declare. I think it's because there is no explicit path for templates.tmpl. I tried different options such as this https://github.com/helm/charts/issues/15375 but that didn't help.

To be honest I don't care whether I am declaring that variable in an external template file or in the YAML code. Whatever works.

Thanks very much for your help.