Addforce in local space?

hey, how do you change a addforce from global to local? heres what i have at the moment

this.rigidbody.AddForce(0,-1000,0);

Instead of it going straight down i'd like it to go along its -z axis. :)

Use AddRelativeForce instead of AddForce.

Searched for hours to get an answer this easy -_-
Thanks!