How to mask a website URL transparently redirecting in Windows Server DNS

I am trying to setup a SharePoint intranet site in Office 365. Our DNS provider doesn't support masked redirects like GoDaddy does.

Good for them! "Masked redirects", as GoDaddy calls it, are not a DNS function but rather a simplified HTTP redirect or reverse proxy. As a result DNS providers are under no obligation to provide that functionality.

Masked redirects maintain the original requested domain in the browser URL bar.

That's because they're HTTP redirects and not a function of DNS.

I have a split DNS configuration in my environment and want to do this with my Windows Server running DNS services.

Awesome! However that's completely irrelevant to this situation since DNS does not perform HTTP redirects. Are you seeing a theme yet?

How can I set up a masked redirect in Windows Server 2008 using DNS?

You can't do that with DNS. Surely now you're seeing a pattern.

I have tried doing it with a cname record but it doesn't mask the domain.

That's because you can't perform "masked redirects" (or whatever GoDaddy calls it these days) with DNS. GoDaddy is using HTTP reverse proxies and masking all the hard work away from you. You'll need to use either an HTTP redirect to get people to the other site, or a reverse proxy to pull from the other site and present it as if it was the URL that you want.

Oh, also: DNS can't do HTTP redirects or reverse proxying.

_{Side note: DNS doesn't do HTTP redirects or reverse proxying.}