Thread:Extension talk:InputBox/Placeholder support?/reply

It should already support placeholder attribute since this commit: https://gerrit.wikimedia.org/r/gitweb?p=mediawiki/extensions/InputBox.git;a=commitdiff;h=6b49e7d6d899dfe276c6993d53874475e7bedb5e

If you're using an up to date version, you just need to use. :)