Source

File: wp-includes/class-wp-customize-widgets.php

public function get_setting_id( $widget_id ) {
		$parsed_widget_id = $this->parse_widget_id( $widget_id );
		$setting_id       = sprintf( 'widget_%s', $parsed_widget_id['id_base'] );

		if ( ! is_null( $parsed_widget_id['number'] ) ) {
			$setting_id .= sprintf( '[%d]', $parsed_widget_id['number'] );
		}
		return $setting_id;
	}