Right click Livesplit -> Open Splits -> From File

Right click Livesplit -> Edit layout -> Remove "Splits" -> Add "Subsplits" (under list)

Uses the default config of the splits.txt file from SpeedrunUtils:

-- Any%
Prologue End,true
Early Mataan (Splits when you enter Millenium Square),false
Versum Hill Start,true
Dream Sequence 1 Start,true
Chapter 1 End,true
Brink Terminal Start,true
Dream Sequence 2 Start,true
Chapter 2 End,true
Millenium Mall Start,true
Dream Sequence 3 Start,true
Chapter 3 End,true
Flesh Prince Versum End,false
Flesh Prince Millenium End,false
Flesh Prince Brink End,false
Pyramid Island Start,false
Dream Sequence 4 Start,true
Chapter 4 End,true
Final Boss Defeated,true

Paste into splits.txt and use this splits file.