Assuming you have a configured and ready-to-use iSCSI target, you can use the following snippet to attach it to a VirtualBox VM.
VBoxManage storageattach "ubuntu" --storagectl "SATA" \ --port 0 --device 0 --type hdd --medium iscsi --server 10.1.1.202 \ --target "iqn.2005-10.org.freenas.ctl:freenas-iscsi-target" \ --tport 3260 --lun 0
In the above snippet:
I had issues running Ubuntu in legacy boot mode using iSCSI but switching to UEFI made it work.