Creating a Text Field

Trying to create a keypad with a text field above it so that when the correct code is put in an event takes place. In this case my alarm panel will be disarmed. I am running into the issue that when I use the property action and set the text of the field to the number of the button I pressed it replaces the number that was previously inputted. How can I set the property action to add to the text field (which is just a plain label) without replacing the number already there?