How to detect when a RadioButtonList is clicked?

Hi, guys I am here again.
How are you?
As always, I have a dificult problem.Now I have two radionlistbuttons and when I click in the first i need to put a value in  a textbox and when i click in the order i need to put another value in the same textbox.
I need to detect when the radiobuttonlist is clicked?
Is it possible?
Thanls a lot guys?
Regards 
 
 
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