Is the World Name in the Advanced options the World Seed?
If not how can I find it or change it?

Valheim is a SUPER new service offered here, and as such the community (Well, me mostly.) is totally inexperienced with the panel or the game. (I still don't own it, as I'm waiting for the "hype period" to expire.)

I'd recommend submitting an email for official support.

Hi Alfcon, You can create a new world local to you machine and specify the seed you want. Once the world is created you can star your server once and stop it. You should then find the world folder on the FTP where the worlds are stored. Copy your World from the local PC to the server and rename the world in the config. Done you can now play that seed. If you need  more detailed version of this let me know :)