Drag and drop into your level.
UE uses post processing at the end of its rendering process. You can change the post process settings via a post process volume. By default a post process volume only affects the area it encompasses. By setting it to unbound it will affect the whole level. The practice I use is to place one unbound post process volume to affect the whole level and then place more if I want to have different post process settings in certain areas.
Keep in mind that UE always uses post processing regardless of whether you have a post process volume or not. Post process volumes are only used to override the default settings.
Once you’ve placed a post process volume into the level, select it and check “Infinite Extend (Unbound)” in its settings.
doc on post process volume: Post Process Effects | Unreal Engine Documentation
doc on bloom: Bloom | Unreal Engine Documentation