1. Create a page, outside of the Blackbaud NetCommunity, with a static URL
2. Add the link to the third party website to the new page
3. Create an IFrames part that points to the new page
4. Add that IFrames part to a new Blackbaud NetCommunity page

When some one loads the Blackbaud NetCommunity page it will show the link housed on the external page. When they click the link, it will transmit the URL of the page that it is contained in, not the Blackbaud NetCommunity page URL.