Within your widget, you can include a editor with a textarea and the class custom-widget-wp-editor
.
<textarea id="<?php echo $this->get_field_id( 'header' ); ?>" name="<?php echo $this->get_field_name( 'header' ); ?>" class="custom-widget-wp-editor"><?php echo $header; ?></textarea>
Make sure you include the JS-file within the widgets and customizer view.
/**