Hey there!
Each Contact Point will have fields listed under the “Optional Settings” dropdown that support message templates, as in the example shown here:
In that example, the template named “mymessage” is referenced in the Text Body field:
{{ template "mymessage" . }}
Not all Contact Points will have the same fields available for customization, but all use the Go templating language.
For the settings available for the Webhook Contact Point for example, if you created a message template named “default.username” to customize the username field, you would reference that as:
{{ template "default.username" . }}
Related docs for easy reference:
Note that the Contact Points and the optional settings for the Contact Points will also vary depending on the type of Alertmanager. For the differences between the two types of Alertmanagers, see this post.