How to implement this rule for default site routing?

I have a situation where the domain name internal is the same as the external website of the company so I want to avoid changing the domain name to internal.domain.com but instead I want to try to use this:

<rewrite><rules><rule name="CanonicalHostNameRule1"><match url="(.*)" /><conditions><add input="{HTTP_HOST}" pattern="^www\.domainName\.com$" negate="true" /></conditions><action type="Redirect" url="http://www.domainName.com/{R:1}" /></rule></rules></rewrite>

It says on the website to use this code in the web.config file but IIS 8 doesn't seem to have such a file. Where can I implement this?

Please advice as this issue is really annoying to the company as whenever they want to access their external website it points them to the IIS page. 

Thanks a lot.