Use subdomain for S3 assets in multi-site Craft installation I have found the section in settings where I can set the URL Prefix for an asset source using a CNAME and pointing to the relative S3 bucket where the file actually lives but I am using a multi site environment so I need the value of the URL Prefix to be dynamic.

I have tried setting it to assets.{{ craft()-&gt;request-&gt;getServerName() }}, also have tried assets.{% craft()-&gt;request-&gt;getServerName() %}but that is what's getting spit out on the page (asset url as rendered on page looks like assets.{% craft()-&gt;request-&gt;getServerName() %}images/home-banner.jpg. 

How can I make Craft parse this field's dynamic value? The statement isn't parsed as it would be if it was in a Twig template.
Your example of setting the S3 "URL Prefix" setting to something dynamic won't work because there are only a few settings in the Control Panel that Craft parses for dynamic Twig content and that isn't one of them.  i.e. in an Asset Field's Upload Location, Section Title/URL Formats, etc.

Honestly, I don't think there currently is a way to do that in Craft 2.

This should be possible in the upcoming Craft 3 by overriding the setting in config/volumes.php and doing your dynamic logic from PHP.