This depends on what OS the end users are using, followed by their IE Version. You can run a GPO to force user to use a proxy, you may want to read this.
Ideally, you may also just have a login script that does something similar to:
Regedit4
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings] "MigrateProxy"=dword:00000001 "ProxyEnable"=dword:00000001 "ProxyHttp1.1"=dword:00000000 "ProxyServer"="http://ProxyServername:80" "ProxyOverride"=""
You can read up on this here.