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 you can upload the world to your server. --> [Upload Valheim Save]

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.