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You can define spawn using WorldEdit or other plugins such as EssentialsSpawn, although WorldEdit + WorldGuard does an amazing job.
You can do this by using a wooden axe and selecting two opposite corners of your build, this will select a box around your build.
Then you can do /rg create spawn followed by /rg flags to customise your region (people will automatically not be able to build here) If players have a large spawn radius or spawn isnt in the area you want it to, you can use /gamerule spawnradius 0 and Essentials has /setspawn to set the block you are stood on to the block players will spawn at.
Good to know: You can create a support ticket, describe your problem and, send proof of purchase so that you can be provided with the correct world download