It is explained here: https://pve.proxmox.com/pve-docs-9-beta/chapter-pvesm.html#storage_lvm
Simply put, you need to manually add the option in the storage.cfg file yourself. eg:
A LUN exported from a Qnap server
iscsi: qnap
portal...
My plugin is out of beta and is operational. I've also developed a fully API driven that will be available for TrueNAS 25.10.
https://github.com/boomshankerx/proxmox-truenas