how can i mark a function as an Onto function

I just remember it vaguely from linear algebra but it's like this (kind of weird since we went over it quickly in less than one lecture and that's it): 
http://www.regentsprep.org/Regents/math/algtrig/ATP5/OntoFunctions.htm 

I am not sure if this calculator can map onto functions.

You are one up on me Aziz, I don't know what an onto function is.