Did you ever figure out how to do this, Chris?
austin.openshaw commented here in Open Internal Links in new tab/window that dropping the 's' from https in the internal link makes it open in a new tab as it is now treated as an external link provided you have configured external links to open in a new in admin console of your community.
fwiw I found another solution which was to add the following to the html href...
In action here... 3rd button - https://community.canvaslms.com/community/ideas/groups ...
<a href="https://docs.google.com/a/instructure.com/forms/d/18Qdc9piOyn5NdVSjD1h09nnVf845LZPIxZWdXuFmki" target="_blank"><img class="fullWidth" src="/resources/statics/1523/button%2BSuggest%2Ba%2BGroup.png"></a>