For a http proxy
-Dhttp.proxyHost=PROXYIP -Dhttp.proxyPort=PROXYPORT
For a socks proxy
-DsocksProxyHost=PROXYIP -DsocksProxyPort=PROXYPORT
For a socks proxy with authentication
-DsocksProxyHost=PROXYIP -DsocksProxyPort=PROXYPORT Djava.net.socks.username=PROXYUSERNAME java.net.socks.password=PROXYPASSWORD
PROXYIP being the IP of the proxy
PROXYPORT being the port of the proxy
PROXYUSERNAME being the username of the proxy
PROXYPASSWORD being the password of the proxy
and just start powerbot with a batch file or from cmd