Could the proxy dialog have a “Use system proxy settings” option, as in Firefox (search Firefox’s settings for “proxy”)? Typing in, say, “127.0.0.1:9050” does not work.
If you want to use SOCKS5 proxy, type in socks5://127.0.0.1:9050
As for “Use system proxy settings”, it is technically possible, but I don’t know how exactly it works across operating systems. Some use somewhat standardized environment variables (but IIRC it varies a bit even between Firefox and Chrome how they are interpreted), but Windows and macOS may have some other mechanisms.