That's not possible as far as I know. Only corporate ISPs who provide a branded & customised web browser can do that.
It can only work if each user sets your company website as their "Home Page" in their Internet Explorer: http://windows.microsoft.com/en-gb/internet-explorer/change-home-page#ie=ie-11
But they could just easily set the home page to something else if they get fed up with seeing your website.
There is no way you can "lock it down" to prevent users from changing it since it has nothing to do with your router setup or configuration. You can't even force them to use Internet Explorer for that matter.
What they use and what they see on their own computer is their business, not yours.