publish Internal simple Http site to Internet and force external client to preauthentication themself

Hi

I have a http site in my internal LAN domain , recently manager ask me to publish the web site to internet for our external employee .

--the web site is http and its simple site without any preauthentication .

I Want to publish it in the way that make external user to force preauthentication and after that they can access the web site and prefer to  use SSL  .

my external user just want to use web browser .(no vpn connection or others...)

thanks