Custom Monsters

I’ve been making a Bounty Hunter monster, I have most things (hopefully) completed, I just need to test the monster itself. Though, where can I adjust where it spawns? Like try to make it spawn in the Weald only, or all areas?

EDIT: I know how to put the monster in an area, but I get this error,

Assert Failed!
(region !=nullptr
SpinedataBinaryLoader::ReadAttatchment in spinedata_binary_loader.cpp line 349
couldn't find region!

Esc to skip

After the error, the game crashed.