Hey guys! You have to wait for that a little bit (i guess?). First the developers must fix the game, so it have stable options for server to be hosted, and all the problems that comes with it.
This game is still in development, and just now it is not possible, due to the coding of the game.
I have been in contact with the developers, and I have been giving them a few pointers.
AFTER everything is fixed in the game, and server-hosting and bugs, and everything, the game must be a little famous, right?
A server provider like Nitrado can't just host all games that is out there, no server provider can, since it is a ocean of games on the market.
This is a guess from my side, but I belive that the game must have both a stable coding, and some sort of popularity, before the add it to thier option.
I am also a big fan of Valheim, so I am on your side guys, but this will take some time I belive...
* hugs from me anyway *
meanwhile all the other bigger hoster got valheim up and running since day 2