You have a VPN Server installed outside of your Mikrotik Local Area Network. The VPN Server can be in your premises or outside premises. You have a username and password for connecting to that external VPN Server thorugh your laptop or desktop. To avoid providing username and password to all local area network users you want to create VPN client in Mikrotik router through a username and password that you use to connect on your computer to route all Mikrotik LAN Users to connect to specific website using that particular VPN connection. Here are the configuration steps:
/interface pptp-client add add-default-route=yes connect-to=IP Address of VPN Server disabled=no name="External VPN" \ password=password user=username /ip firewall nat add action=masquerade chain=srcnat out-interface="External VPN" src-address=\ 192.168.2.0/24
Where 192.168.2.0/24 is your Mikrotik LAN Network.