c# - ASP.net Textbox that adds "ING" -

i have textbox verbs , want auto add "ing" word user tries typing in box if not there. possible? , how?

my verb textbox is:

tbverb 

think should want.

if(textbox.text.substring(textbox.text.lenght-3) != "ing")    textbox.text += "ing"; 

but need validate or event guy in other answer suggests.