Posting this in case it helps anyone in the future. I needed to pre-test an HTTP header that Akamai sends, “True-Client-IP”, and the following script does that, virtually defining the location of the client visiting a site. Enter the script under Advanced Settings, replace the bogus IP and domain, and run your test. My site found the added header and determined the origin based on the value in True-Client-IP. Very cool! Thank you WebPageTest!
addHeader True-Client-IP: 123.456.789.012