Why not, just use the socks 5 proxy it may give you better privacy and security.
Never heard that you can add HTTP proxy in electrum even lower version.
You should be aware also on fake electrum website like the listed from here
⚠⚠️⚠~Beware on active phishing Electrum websites~⚠⚠️⚠Also, beware on fake Electrum wallet software version 4.0 Electrum doesn't release this version and the current latest version is Electrum-3.3.4 and the original website is electrum.org