I am trying to do a mock of a login screen with a PIN number. I want to concatenate the number in a text box as the user presses the numbers. For example, if the user presses 1, I want the text box to say 1. If the user then presses 5, I want the textbox to concatenate the values so now the text box reads 15. If the users presses 7, I want the textbox to read 157.