Well, Jetty documentation 
https://www.eclipse.org/jetty/documentation/9.4.x/http-client-proxy.html
Maybe I missed that part that it can be done in the configuration somewhere, but in the link above it looks like it needs to be done in the code … but maybe I’m wrong about that … I agree that the parameters are for Java, so maybe it should work? Did you find a page where it states the parameters do work with Jetty?
Note; lightbender is on Windows 7
Regards
Beat