Open external webpage on post click?

First you need to install the plugin https://www.ads-software.com/plugins/page-links-to/ which will allow you to make a click on a post (title, read more or direct request) redirect to any other location you like.

Next, you’d need to make it so that these kind of links on your site carry a class="fancybox-iframe" attribute. You might be able to do that by modifying your theme or by adding some javascript that detects linked post titels and read more links that have an href with an external domain.

This all depends on your theme…