Hi Juno. You need to do the following.
Stop your server
FTP into your server, the FTP information is in your control panel.
In the main directory is a file that ends with .FWL, download it or open for edit
You can copy the hex code and paste into a site like https://hexed.it/ which will show the seed as part of the string, or
You can drop the FWL file into the seed extractor here -> http://valheim-map.world/getseed.html
Close FTP and start your server back up
I found a very easy and simple way to join my server and for others to join. Goto toparkservers.com
Enter your server name in the search box, wait for it to list if running, follow directions to run game. Runs the client thur Steam. Works great for me.