hugo-theme/layouts/_default/_markup/render-link.html

{{- $sameSite := true -}}
{{- $finalDest := .Destination -}}
{{- if eq (.Destination | relURL) (.Destination | absURL) }}{{ $sameSite = false }}{{ end -}}
{{- if not $sameSite -}}
    {{- if in $finalDest "?"}}{{ $finalDest = printf "%s%s" $finalDest "&utm_source=indieweb" -}}
    {{- else }}{{ $finalDest = printf "%s%s" $finalDest "?utm_source=indieweb" -}}
    {{- end -}}
{{- end -}}
<a href="{{ $finalDest }}"
    {{- if not $sameSite }} target="_blank"
        {{- with .Title }} rel="{{ . | safeHTML }}"{{ end }}
    {{- else }} rel="me"{{ end }}>{{.Text | safeHTML}}</a>