[Masterful1]

In the Admin Applications guideline, you would find this

Steam ID : If you do not know this, create a server from within the CS:S start menu. Then type status in the console to get your steam ID. Example: STEAM_X:XXXXXXX

Even non-steam players have a SteamID. Look at the other applicants to see the format their SteamID is in so you know what you're looking for.