/*
 * add custom field to WP widget form
 */
add_action('in_widget_form', function ( $widget, $return, $instance ) {
	echo 'This Is visible in Elementor too.';
	$my_custom_field = isset( $instance['my_custom_field'] ) ? $instance['my_custom_field'] : '';
    ?>
	<p>
		<label for="<?php echo $widget->get_field_id( 'my_custom_field' ); ?>"><?php _e( 'My custom field:' ) ?></label>
		<input type="text" class="widefat" id="<?php echo $widget->get_field_id( 'my_custom_field' ); ?>" name="<?php echo $widget->get_field_name( 'my_custom_field' ); ?>" value="<?php echo esc_attr( $my_custom_field ); ?>"/>
	</p>
    <?php
}, 10, 3 );

/**
 * save custom field of WP widget form
 */
add_filter( 'widget_update_callback', function ($instance, $new_instance, $old_instance, $widget ){
	$instance['my_custom_field'] = isset( $new_instance['my_custom_field'] ) ? sanitize_text_field( $new_instance['my_custom_field'] ) : '';
	return $instance;
}, 12, 4 );