Pressing "Start" on your server will auto create a world but if you want to add to your world Follow the step:
1.) Go on your server
2.) Press file
3.) Upload your world file
<Example:Coolworld.wld> <<example this is your world file name
4.) After uploading your file press "Sittings"
5.) Find the "world name" and rename it to your world name <Example:Coolworld.wld> this is your world file name The wolrdname must be "Coolworld" the ".wld" was removed to run the world
6.) Restart your server
✔️Done✔️