/**
 * Appends the preview token to the given URL as a query parameter.
 * If the preview token is not provided, the original URL is returned.
 *
 * @param url - The original URL.
 * @param previewToken - The preview token to append.
 * @returns The URL with the preview token appended as a query parameter.
 */
export declare const appendToken: (url: string, previewToken?: string) => string;