How to remove all chars after a space in Textbox

My Problem is that my Users sometimes did mistakes and i want to avoid this by removing the splitted part of the string automatically.

i dont want to combine the splitted string, i just want to remove all what is written after the space.

Should look like this:

All what i selected should be automatically removed after i press the Button to Save/Confirm.

>Solution :

Source: Removing everything after the first " " in a string? (space)

It would be simpler to do a string.Split():

string input = "some:kind of string   that I want      totrim please."; 
input = input.Split()[0];
//input = "some:kind" now

So it should look something like this, based on what are you working on (I suppose WPF)

string inputText = txtInput.Text;
inputText.Split()[0];