Your project (the .blend file) must be saved on disk for this button to become active.
Export to FBX with autocorrection. Does not change anything in the open blend scene or file.
- corrects the scale (no need to mess with the scene unit scale)
- corrects the bone rotations so they match the Mannequin origin (aka chiropract)
- generates IK bones (if they don't exist yet)
- autocorrects the armature root name ("root" in UE4 skeleton)
- Exports the result into FBX
File -> Export -> FBX (Chiro UE4 Mannequin) -> Save as ...
You only choose the file name. The folder is of the original blend file.