To authenticate proxies with username and password on Selenium, the most common approach on various programming languages is to write a custom extension which would handle the proxy connection. Such an extension is shown in extension.py example.
The proxies function in the provided extension.py file can be imported and easily used in any project as given in the main.py example. The function expects 4 values: username, password, endpoint and port.
proxies_extension = proxies(username, password, endpoint, port)
The extension is then added using the add_extension method:
chrome_options.add_extension(proxies_extension)
This example can be easily adapted for use in other programming languages as well as supported webdrivers.