How to Change a Server Seed

Server Seeds

Currently with a dedicated server there is no way to specify a server seed.
If you want to use a custom seed you will want to go into single player and created your world with your desired seed, then following the upload instructions (which can be found on your control panel) upload the world to your server.

Here is a tool that allows some customization in seed creation. [Seed Finder]

If you want to find out what your current server's seed is, you will want to download a copy of your world via FTP and place it into C:\Users\%USERNAME%\AppData\LocalLow\IronGate\Valheim\worlds then boot up your game and you will be able to see the seed from the single player world select menu.