Which function is used to paint terrains with the terrain engine?

I am trying to paint a terrain via C# script. I want to be able to paint grass at certain points on a given terrain, not in the editor, but on runtime. The only thing I need to know is: Where is the actual function that is used to paint the terrain (usually with the brush tool) in unity editor? I know there are other work-arounds but I am specifically interested in this one. Thank you