Supposedly, adding the HTTP_PROXIES parameter to the Netbox config is all that's required. If this doesn't work, leave it in-place and also modify your systemd service file to contain the http_proxy, https_proxy and no_proxy environment variables.
Add the following parameter to /opt/netbox/netbox/netbox/configuration.py - adjust as necessary.
HTTP_PROXIES = {
"http":"http://forward-proxy.exmaple.com:3128",
"https":"http://forward-proxy.exmaple.com:3128",
"no_proxy":"localhost,127.0.0.1,.int.exmaple.com"
}