From Xojo Documentation
The selected radio button has been changed.
Use the SelectedCell method to determine which radio button was clicked. SelectedCell returns a Pair with the Left property containing the row index and the Right property containing the column (both are zero-based).
This code in a default WebRadioGroup tells you which radio button was clicked:
p = Me.SelectedCell
Select Case p.Left
MsgBox("You clicked the top button.")
MsgBox("You clicked the bottom button.")
You can also use SelectedCaption to check for a selected radio button based on its caption: