There should be a Dedicated launcher that looks like the Blockland launcher but is named differently.
When you installed v20 it gave you an option to create a dedicated shortcut.If you choose not to, you just need to create another shortcut, with the target "C:\Program Files (x86)\Blockland\BlocklandLauncher.exe" -dedicated -map mapNameChange the file path to the path to your launcher, and mapName to the name of the map you want.Alternatly you could just copy your existing shortcut and add -dedicated -map mapName to the end of the target