1. Right click on Valheim in your Library, select Properties and add ‘-console -window-mode exclusive' without the quotes to 'Launch Options'.
2. Right click on Valheim in your LIbary again, select ‘Manage’ and ‘Add Desktop Shortcut’ so the console, etc. works in game (just to make sure).
3. Launch Valheim from your new Desktop Shortcut. (hit F5 in game, type 'Devcommands' and spawn all the tasty mead you want).

Cheers