How to link to another site through the SiteMap, like CRM Help Center

I want to add a button to the SiteMap that links to an another URL and I would like the linking behavior to funciton like the Help link provided by Microsoft:

1) The current tab remains the same

2) The new tab contains the linked content

I've tried added a full URL to the sitemap, but this opens the new URL in the iframe.

I've added a web resource, as outline here, which links the nav button to a web resource that uses window.open to open a new tab, but this leaves the originating tab in an undesirable state.

Any ideas?

Thank you.