SteamID and Custom URL
You can find users on AStats based on SteamID64 or on the Steam custom URL.
If you don't know how to find yours, here is a simple way:
1. Goto Steamcommunity
2. Login into your Steam client and go to account details (found at the right top of Steam).
3. Look at the SteamID
4. Copy that and enter it in the field.
Note: Steam profiles must be set to public to be added.