Choosing your server software
The Software tab has one option for Insurgency: Sandstorm — the Public branch, with no alternate or test branches to pick between. Any change here requires a server restart to take effect, which the panel will confirm before applying, so time it for when a match isn't in progress.
Next steps
Once your server is running, use the Console tab to verify a clean startup. See Installing Mods on Insurgency: Sandstorm if you're considering mods — there's no automated mod manager for this game. Back up your server configuration before experimenting — see Managing Backups & Restoring Your Server — and use Scheduling Automatic Server Tasks to automate restarts.