Take the shortcut on your desktop that launches blockland.
Copy it
Paste it somewhere else on your desktop
Rename it Blockland Dedicated Server
Left click on it and go to properties
In the "Target" field go to the end of the string there and add this to the very end, even behind the quotes.
-dedicated -map slate
Run it
???
Profit.
Also you can change the map in the map argument obviously.