I can think of two ways to do this. Not sure if they're the best options, but they work :)

Create a Single Line field and set the default value to the current user name. You can use a Field Rule to hide it and save the value if you don't want the Submitter to see it.

Set the Radio Button default to 'B' (my example has 'Logged In User'). Then add this JavaScript to change it if the submitter is Anonymous.

var formStartedBy = LFForm.getFieldValues({fieldId: 70});
    
if (formStartedBy === 'Anonymous User') {
  LFForm.setFieldValues({fieldId: 107}, {value: "Anonymous User"});
}

Alternatively, if you don't want to set a default value for the Radio Button, you can use this JavaScript.

var formStartedBy = LFForm.getFieldValues({fieldId: 70});
    
if (formStartedBy === 'Anonymous User') {
  LFForm.setFieldValues({fieldId: 107}, {value: "Anonymous User"});
} else {
  LFForm.setFieldValues({fieldId: 107}, {value: "Logged In User"});
}