It should be fairly straightforward. I did this on Linux Mint but the process should be identical for Ubuntu:
Log on to the VPN web UI.
On your dashboard you will see a Devices widget, click on the LogonBox VPN Client for Linux, which should be highlighted in green as the browser should have detected the OS you are logged on from.
This will download a logonbox-vpn-client-unix-amd64-<version>.sh file.
Install that with:
sudo sh logonbox-vpn-client-unix-amd64-<version>.sh
then follow the graphical installer.
This should install a LogonBox VPN item into your menu which you can now launch.
Once launched, enter the server address for your VPN and click connect.
You will be asked to authenticate and the client should connect up.