There are several factors, depending on how you have things set up.
If your internal clients can access the web in general, then it’s just a matter of adding the web server address to the DNS. This will depend to some extent on whether your internal domain name and external domain name are set up. It’s very common these days to run two different DNS domains such as somedomain.local (inside) and somedomain.com (outside). Older setups often split the same domain – two different servers, both hosting the same domain inside and outside, but with different data (and usually some overlap). In those cases, you just add the A records to the internal copy.
If your internal systems CANNOT reach the internet, then a single purpose proxy server is probably your best bet, which would include adding the web page A record to your internal DNS with the IP pointing to the proxy server.