You do not have to add the ID and API key if you're accessing the URL from the same IP as you are hosting the server(s) from
Example URL to get the player count of your servers, in that case, would be: https://api.scpslgame.com/serverinfo.php?players=true
How to use the server list API to get data from your servers from a different IP Address:
Step 1: Head over to your server console and type in !api reset to generate your token. (Or type !api show to show your token if you already made one before)
Step 2: Type !id in your server console to get the server's current Account ID.
After this you can add &query=true to the end of the URL if you'd like that specific piece of information.
The list of available queries can be found here: https://api.scpslgame.com/
Example URL that returns the player count of all your servers: (This is not a valid URL, don't bother clicking on it)