The installer and your licence key come by email when you subscribe, and stay in your Lemon Squeezy order page.
Each version's SHA-256 is listed with the download, so you can check the file you got is the real one. In PowerShell: Get-FileHash ".\Ilm's Rust Cinematic Client Setup 0.3.1.exe".
The installer isn't code-signed yet, so Windows SmartScreen may warn about an unknown publisher. Check the SHA-256 first, then choose More info and Run anyway. Only do this for a file whose checksum matches.
In the app, open Settings > Licence and enter your licence key. It works on 1 of your own computers at a time; to move it, deactivate it in the app on the old one first. The server plugins check the key every time the server starts.
/crate: every openable crate in the game./car: modular car builder./base.