As I said, radio button's event "onChange" is not fired when you load a form.
So you may want to call xyz function in the onLoad event of your form.
onLoad : function(){
ca_fd.js.xyz();
},
xyz : function(){
var radioValue = ca_fdGetSelectedRadio(ca_fd.formId)'radio_name');
if(typeof(radioValue) != "undefined" && radioValue != "")
ca_fdSetTextFieldValue(ca_fd.formId, 'textbox',radioValue);
}
Set onLoad(); on the onLoad attribute of your form.