I have used the Area51 Proposal shares and also the flairs of profile but they don't target new tab or window
I tried to add the
target="_blank" but that doesn't seems to be work
This is not a bug, this is very much status-bydesign; the limited subset of HTML that the Stack Exchange Markdown flavor supports doesn't support specifying a target. This applies to all links, not just Area51 promotions.
See What HTML tags are allowed on Stack Exchange sites?, only
title attributes are permitted.