use Radio Button to change the editcontrol value

Hi,
I added three Radio Buttons in Add Page with HTML Editor,

<INPUT value=RM type=radio name=productType>Raw Material

<INPUT value=CN CHECKED type=radio name=productType>Normal Product

<INPUT value=CS type=radio name=productType>Commerical Sample
and now i want to do like this. when the users click on any of the button, it will change the first two characters in a read only text field.
i tried to write some code in JavaScript OnLoad event but seems can't add my own function there.
can anyone help? thank you.