As root user:
cd /usr/local/openvpn_as/scripts/
./sacli --user "Username Goes Here" --key "access_to.0" --value "+NAT:172.19.0.0/16" UserPropPut
./sacli start
See also:
https://openvpn.net/vpn-server-resources/managing-user-and-group-properties-from-command-line/