Webform custom element Ajax callback

I have already developed a field widget that queries an API based on a value in a textfield and i want to now create a Webform element to do the same thing for poeple using Webform.

Because i need my element to have a textfield, a button to trigger the API request, and an HTML area to display the result, I am presuming i need to extend my class from WebformCompositeBase.

If I write the callback in the same way as I have done on my working field widget, as per below - I get "Error: Using $this when not in object context" error:

  public static function getCompositeElements(array $element) {

    $drupal_translation = \Drupal::translation();

    $elements = [];

        $elements['registration_number'] = [
          '#type' => 'textfield',
           '#title' => $drupal_translation->translate('Vehicle Registration Number'),
        ];

        // Add a button for sending the API request.
        $elements['api_button'] = [
          '#type' => 'button',
          '#value' => 'Check vehicle details',
          '#ajax' => [
            'callback' => [$this, 'sendApiRequest'],
            'event' => 'click',
            'wrapper' => 'api-response-wrapper', // The HTML ID where the API response will be displayed.
          ],
        ];
    
        // Display area for API response.
        $elements['api_response'] = [
          '#type' => 'markup',
          '#prefix' => '<div id="api-response-wrapper">',
          '#suffix' => '</div>',
          '#markup' => '',
        ];

    return $elements;
  }

If I write it like below I get a "The specified #ajax callback is empty or not callable" error.

          '#ajax' => [
            'callback' => 'sendApiRequest',
            'event' => 'click',
            'wrapper' => 'api-response-wrapper', // The HTML ID where the API response will be displayed.
          ],

The sendApiRequest function which is inside the class is as follows:

  public function sendApiRequest(&$element, FormStateInterface $form_state, &$complete_form) {
    \Drupal::messenger()->addMessage('API called');
  }