Super late reply but for reference I was having same issue couldn’t add it.
So this is what I did:
With linear Siren 1 feet away from ST hub:
Go to z-wave utilities in ST app.
Click General Device Exclusion
On the back of the siren alarm i pressed the tamper button and held for 2-3 seconds.
Received a message that an unknown zwave device was reset and could now be added.
I then clicked add a thing on main ST screen and held the tamper switch 2-3 seconds and released.
Item was identified as switch which I then changed the device handler to the one available at [RELEASE] GoControl Siren / Linear Siren / Vision Siren
In general for z-wave items it helps to reset/exclude it by General Device Exclusion first then doing the Add a Thing steps.