/**
 * Adds a string to a custom attribute string array, or creates that array if one doesn't exist.
 *
 * @param key - The identifier of the custom attribute. Limited to 255 characters in length, cannot begin with
 *    a $, and can only contain alphanumeric characters and punctuation.
 * @param value - The string to be added to the array. Strings are limited to 255 characters in length, cannot
 *    begin with a $, and can only contain alphanumeric characters and punctuation.
 *
 * @returns Resolves to a boolean that indicates whether the update was successfully enqueued. Resolves to undefined
 *   if the SDK is not initialized.
 */
export declare function addToCustomUserAttributeArray(key: string, value: string | null): Promise<boolean | undefined>;