This tutorial is going to show you how to put a seed of your choice on your Minecraft server.
Requirement: You will need to have a Minecraft server with the server.properties file.
To use a Specific seed, you will need to delete your old world(s) via the 'File Management' section of the panel.
Afterwards, Select all of your world files and delete them and don't restart the server.
When you delete a world, you cannot get it back unless you have a backup.
After removing all the old world file, you will need to go in the 'server.properties' file and look for the line 'level-seed=' and put the seed you want after the '=' and save it.
After that you will be able to start your server up and have the seed that you wanted on your server.