I have a form with a checkboxSetField list which works as expected.
For each checkbox item in the loop I want to call a method, I know this is out of scope so I have created an extension.

CheckboxSetField::add_extension('OperatorFormExtension');

OperatorFormExtension.php

class OperatorFormExtension extends DataExtension {
    public function testing($ID) {
      return 'hello - '.$ID;
    }
}

checkboxSetField.ss

    <% loop $Options %>
        <input id="$ID" class="checkbox" name="$Name" type="checkbox" value="$Value.ATT"<% if $isChecked %> checked="checked"<% end_if %><% if $isDisabled %> disabled="disabled"<% end_if %> />
        <label for="$ID">$Title, testing('myParam')</label><br>
    <% end_loop %>

When I call the method as above I don't get anything displayed on the page.
If I extend the Form class instead of CheckboxSetField I can call the testing method from the form but still not from the checkboxSetField template.

How do I call a method from the CheckboxSetField.ss template?