How can you choose which input field to write to

[pentan]

Here is the situation.  I've got to text input boxs, a and b.  I have a button that writes something.  I want the button to write to which ever box I have clicked in.  Anyone know how to pass the name of the box that has been clicked on back to a function that writes something? 

[jaronblake]

When you say "clicked on" are you talking about radio buttons, checkboxes, or just clicking the text itself?