Openvpn use the user name and password is the linux server user name and password. When you create a new user in the openvpn gui web page, the corresponding, you need to add a user in linux and set a new password, this password is that you use the new user name connect openvpn services need to fill in the password box.
> adduser newusername ***Omitted*** Copying files from `/etc/skel' ... Enter new UNIX password: Retype new UNIX password: ***Omitted***
Then, open the connection client, use the new account login.