There are two issues here. Every link without protocol (“http://”, “https://”, …) will be considered a relative url to your current localtion by the browser. Opening the link in an new tab is handled by the target option on the anchor element. For most if not all Phoenix.HTML functions options being passed, which do not have a special meaning to that function, will be used as attribute on the generated html tag.
<%= link "Hola", to: "https://www.google.com", target: "_blank" %>