Open External Menu Links in a New Tab

Simple approach

Use a regular Markdown link in content:

[VS Map](https://vsmap.endershollow.com){target="_blank"}

Theme override (advanced)

Create layouts/partials/menu.html and render target="_blank" when a menu URL starts with http:

{{ range .Site.Menus.main }}
  {{ if strings.HasPrefix .URL "http" }}
    <a href="{{ .URL }}" target="_blank" rel="noopener">{{ .Name }}</a>
  {{ else }}
    <a href="{{ .URL }}">{{ .Name }}</a>
  {{ end }}
{{ end }}