Alertmanager with Custom URL within Slack

Question

So I have Alertmanager running with Button in slack. I've created a new button that I want to use with a custom domain, however add an expression to the url based on the alert.

    {{define "__alert_button_link" -}}
       https://example.com/$expression
{{-end }}

So if a high cpu alert came in, the botton would take me to https://example.com/HighCPU or whatever the alert name was.

Thank you.