If you create a new input box. Then it will give you the option of which variable it is linked to. If no variables exist it will allow you to create a new one. Otherwise, double click or add the variable from the left to make it appear on the right. Now when you enter in a value into the input field it will be store to that variable.
Generally you can access variables by using = $(VariableName) or just the VariableName itself.
