In C#, HttpClient takes a handler with the proxy option and credentials with the username and password from the order.
Before opening the connection
Build the handler, set the proxy with the address and port 20001, and pass the handler into the HttpClient constructor. For SOCKS5, use port 20481 with the right proxy type.
A common detail: HttpClient reuses connections. For each task to leave through a different address, create one client per address.