Getting a Player ID
Valheim Player IDs are needed to ban, whitelist, or admin a player and can be found a couple of ways.
F2 Screen
If the player you need the ID of is online, you can use the in-game F2 Screen to get a list of all players currently online and their ID. Just log in to the game and press F2 on your keyboard, this will open a screen in the top left of your game that looks similar to the one on the right. The currently online players and their IDs can be found in the bottom section of the window titled Players.
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Console
If the player you need the ID of is not currently online then you can find their ID in the console logs. Mixed in the debug information that gets printed when a player joins is their Valheim ID and the name of the Character they logged in with. By locating their character name in the logs you can parse out the join event and find their Valheim Player ID.
PlayFab socket with remote ID playfab/**************** received local Platform ID Steam_76561198068595691
Got handshake from client playfab/****************
Network version check, their:12, mine:12
Server: New peer connected,sending global keys
Got character ZDOID from Naspen : **********:*
In the above example, the character Naspen is logging in with the Valheim Player ID, or "local Platform ID" of Steam_76561198068595691. The order of these lines should always be the same, so after finding their character name as part of the Got character ZDOID from line, you can find the PlayFab socket with remote ID line a few lines above with that player's Valheim Player ID.
At this time the console is not saved to a file by Valheim itself or the WinterNode panel, so you'll only be able to see the last few dozen lines of logs.