Here you can see the configuration options for all compatible VPN types. We assume that you are familiar with adding a new VPN connection.
The tests were done with Ubuntu 18.04 and network-manager-stronswan installed, Ubuntu only supports OpenVPN and PPTP with the default install.
It can be installed using the following command on the command line:
apt install network-manager-stronswan
Download the Root CA from the OPNsense Firewall since it is needed for all EAP types with IKEv2.
Open the network manager and add a new VPN connction. Choose IPSec/IKEv2, enter a Name and set the Address to the FQDN matching the one of the certificate at your Firewall.