WebRadioGroup.SelectByTag
From Xojo Documentation
Method
Selects the radio button with the passed Tag. If no radio button has the passed tag, SelectByTag does nothing.
Example
The following code in the Open event of the control assigns the tags to the two radio buttons in a default control.
The following line in a WebButton uses selects the second radio button using SelectByTag.
RadioGroup1.SelectbyTag("Bottom")