Actually, it should be used automatically, after it has been configured via NetBeans.
You could try to add the proxy settings as system properties, either to gpt.vmoptions in the bin directory or tosnap.conf in the etc directory of the SNAP installation folder. Depending on what you call.
This page might help.
https://docs.oracle.com/javase/6/docs/technotes/guides/net/proxies.html
Sorry for not being more helpful. I’m not very familiar with these proxy settings.