Create a file layouts/_default/_markup/render-link.html:
<a href="{{- .Destination | safeURL -}}"{{ with .Title}} title="{{- . -}}"{{ end }}{{ if strings.HasPrefix .Destination "http" }} target="_blank" rel="nofollow noopener noreferrer" {{ end }}>{{- .Text | safeHTML -}}</a>
Alternative, use if you get a buggy HTML output (if empty paragraphs are inserted when using Shortcodes for example)
<a href="{{ .Destination | safeURL }}"{{ with .Title}} title="{{ . }}"{{ end }}{{ if strings.HasPrefix .Destination "http" }} target="_blank" rel="nofollow noopener noreferrer" {{ end }}>{{ .Text | safeHTML }}</a>
Done!
Source: Inspired by Elliot Sachs.