Problem about Radio Button on the Dynpro

Dear friends

I had a problem about radio button on the screen.

The layout of the screen is quite similar to the demo program:DEMO_DYNPRO_CHECK_RADIO.

I try to add function code to the 3 radio buttons. But it is impossible to add it. The field is not inputable.

And, every time before my screen is loaded.

I clear all the values in the field to make it ready for input. However, all the 3 radio buttons are filled.

Who could help me with the overwhelming problem...?

Many thanks