POS (Point Of Sale) Software on c# windows form

How can i add textbox1_textchanged event for a textbox which works for 2 forms or dual monitor means when i am working on 2 Forms (one is for user entry(User_Form) and second is for customer display(Customer_Form)), the textbox1_textchanged event fires when start typing on user's screen(User_Form) textbox and textbox2 text should be changed on customer display textbox(Customer_Form).