i Don't think this is possible im mcreator at the moment (2020.5), but yo ucould makea feature requests on the mcreator github: https://github.com/MCreator/MCreator/issues to allow mobs to spawn in "mob areas" for example wither skeletons in nether fortresses.
just place the enemy in the structure
note: don't forget to select the mob box in structure and make the mob not disappear
i am not making custom structures i am want make with vannilla ones
i Don't think this is possible im mcreator at the moment (2020.5), but yo ucould makea feature requests on the mcreator github: https://github.com/MCreator/MCreator/issues to allow mobs to spawn in "mob areas" for example wither skeletons in nether fortresses.