Config file values removed

[vaporlock]

I'm trying to configure my OpenVPN installation to use a proxy server so I get prompted to enter my proxy credentials.
The config file is here C:\Users\<myid>\openvpn-connect.json

By right clicking on the system tray icon and setting the proxy server and tcp protocol, I get this proxy line:
"http_proxy": "proxy.server.com:8080",

I don't get prompted to enter my credentials. So based on this how to (https://openvpn.net/index.php/open-sour ... .html#http, I added stdin ntlm so it looks like this:

"http_proxy": "proxy.server.com:8080 stdin ntlm",

When I try to connect, I don't get prompted and the "stdin ntlm" portion gets removed.
I'm thinking it's a bad syntax in what I added - does anyone have a working example of getting prompted for proxy credentials?
TIA.