In System Proxy Settings, the software supports only the basic proxy authentication.
You can configure the authentication in the daemon process (mfetpd) by defining the environment http_proxy or https_proxy as a variable.
For example:
http_proxy="http://username:usernamepassword@sampleproxy.url:5678https_proxy="https://username:usernamepassword@sampleproxy.url:5678
Define these environment variables in the deamon process's init script. The configuration process varies according to the distribution you are using in your environment. The software supports these 3 types of init systems:
Systemd
SysVInit
Upstart