For how many years have we been asking for this feature? Yes, you may get this function by right clicking and selecting "open in new tab" or as explained elsewhere for IE (and presumably Edge), and chrome. But an easy identifier is needed when it is obvious that the link needs to open on a page located at a different website (thus taking a person away from Wikitree).
A very verbose solution is to simply indicate it in the text associated with the URL, but this again involves special operations by the browser user. It should be a single click solution.