/**
 * Constructs a threaded conversation ID by appending `;messageid={messageId}`
 * to the conversation ID.
 *
 * In a channel, sending to the result posts a thread reply to the message `messageId`.
 * `App.reply()` does not use this form: it posts through the `replyChains` endpoint and throws when the service rejects it.
 *
 * @param conversationId the conversation to thread into (e.g. `19:abc@thread.skype`)
 * @param messageId the thread root message ID (must be a non-zero numeric string)
 * @returns the threaded conversation ID (e.g. `19:abc@thread.skype;messageid=123`)
 */
export declare function toThreadedConversationId(conversationId: string, messageId: string): string;