How to Add an External Alertmanager to Grafana

How can I enable it to choose alertmanager?
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have you installed the alertmanager datasource plugin? You need this to use an external alertmanager:

Here are the docs. And here is a snippet from them:

Grafana alerting added support for external Alertmanager configuration. When you add an Alertmanager data source, the >Alertmanager drop-down shows a list of available external Alertmanager data sources. Select a data source to create and manage alerting for standalone Grafana Mimir or Loki data sources.

Select Alertmanager

You can configure one or several external Alertmanagers to receive alerts from Grafana. Once configured, both the embedded Alertmanager and any configured external Alertmanagers will receive all alerts.

You can do the setup in the “Admin” tab within the Grafana v8 Alerts UI.