If you want command line, you can use PAM or LOCAL authentication. PAM is when the user credentials are stored in the OS, using standard tools like adduser, deluser, passwd, and so on. With LOCAL authentication, the users exist purely in the Access Server database. Password management can then be done through the web based interface. It can also be done on the command line using sacli and confdba in the /usr/local/openvpn_as/scripts/ folder. See
https://docs.openvpn.net/access-server/ for command line parameters.