WPF change another application textbox

I am looking at some application with snoop.

I can see the text of some textbox, I also can edit it.

Now I want to do the same thing but programmatically. I want to write some text in this textbox programmatically. (Text box from another application)

I think this is possible because snoop can do it (May be using reflection in some way)

What is the best way to do this? Thank you,