diff --git a/apps/meteor/app/lib/server/functions/processWebhookMessage.js b/apps/meteor/app/lib/server/functions/processWebhookMessage.ts
similarity index 69%
rename from apps/meteor/app/lib/server/functions/processWebhookMessage.js
rename to apps/meteor/app/lib/server/functions/processWebhookMessage.ts
index 1e5c34b287bb..4dafc6211015 100644
--- a/apps/meteor/app/lib/server/functions/processWebhookMessage.js
+++ b/apps/meteor/app/lib/server/functions/processWebhookMessage.ts
@@ -1,15 +1,47 @@
+import type { IMessage, IUser, RequiredField, MessageAttachment } from '@rocket.chat/core-typings';
 import { Meteor } from 'meteor/meteor';
 import _ from 'underscore';
 
+import { ensureArray } from '../../../../lib/utils/arrayUtils';
 import { trim } from '../../../../lib/utils/stringUtils';
 import { SystemLogger } from '../../../../server/lib/logger/system';
 import { validateRoomMessagePermissionsAsync } from '../../../authorization/server/functions/canSendMessage';
 import { getRoomByNameOrIdWithOptionToJoin } from './getRoomByNameOrIdWithOptionToJoin';
 import { sendMessage } from './sendMessage';
 
-export const processWebhookMessage = async function (messageObj, user, defaultValues = { channel: '', alias: '', avatar: '', emoji: '' }) {
+type Payload = {
+	channel?: string | string[];
+	roomId?: string | string[];
+	text?: IMessage['msg'];
+	msg?: IMessage['msg']; // overridden if text is present
+	username?: IMessage['alias'];
+	alias?: IMessage['alias']; // overridden if username is present
+	icon_emoji?: IMessage['emoji'];
+	emoji?: IMessage['emoji']; // overridden if icon_emoji is present
+	icon_url?: IMessage['avatar'];
+	avatar?: IMessage['avatar']; // overridden if icon_url is present
+	attachments?: IMessage['attachments'];
+	parseUrls?: boolean;
+	bot?: IMessage['bot'];
+	groupable?: IMessage['groupable'];
+	tmid?: IMessage['tmid'];
+};
+
+type DefaultValues = {
+	channel: string | string[];
+	alias: string;
+	avatar: string;
+	emoji: string;
+};
+
+export const processWebhookMessage = async function (
+	messageObj: Payload,
+	user: IUser & { username: RequiredField<IUser, 'username'> },
+	defaultValues: DefaultValues = { channel: '', alias: '', avatar: '', emoji: '' },
+) {
 	const sentData = [];
-	const channels = [].concat(messageObj.channel || messageObj.roomId || defaultValues.channel);
+
+	const channels: Array<string> = [...new Set(ensureArray(messageObj.channel || messageObj.roomId || defaultValues.channel))];
 
 	for await (const channel of channels) {
 		const channelType = channel[0];
@@ -69,7 +101,7 @@ export const processWebhookMessage = async function (messageObj, user, defaultVa
 			messageObj.attachments = undefined;
 		}
 
-		const message = {
+		const message: Partial<IMessage> & { parseUrls?: boolean } = {
 			alias: messageObj.username || messageObj.alias || defaultValues.alias,
 			msg: trim(messageObj.text || messageObj.msg || ''),
 			attachments: messageObj.attachments || [],
@@ -91,7 +123,7 @@ export const processWebhookMessage = async function (messageObj, user, defaultVa
 
 		if (Array.isArray(message.attachments)) {
 			for (let i = 0; i < message.attachments.length; i++) {
-				const attachment = message.attachments[i];
+				const attachment = message.attachments[i] as MessageAttachment & { msg?: string };
 				if (attachment.msg) {
 					attachment.text = trim(attachment.msg);
 					delete attachment.msg;
diff --git a/packages/rest-typings/src/v1/chat.ts b/packages/rest-typings/src/v1/chat.ts
index b8def106c78a..c29e420a47f3 100644
--- a/packages/rest-typings/src/v1/chat.ts
+++ b/packages/rest-typings/src/v1/chat.ts
@@ -697,8 +697,8 @@ const ChatGetDeletedMessagesSchema = {
 export const isChatGetDeletedMessagesProps = ajv.compile<ChatGetDeletedMessages>(ChatGetDeletedMessagesSchema);
 
 type ChatPostMessage =
-	| { roomId: string; text?: string; alias?: string; emoji?: string; avatar?: string; attachments?: MessageAttachment[] }
-	| { channel: string; text?: string; alias?: string; emoji?: string; avatar?: string; attachments?: MessageAttachment[] };
+	| { roomId: string | string[]; text?: string; alias?: string; emoji?: string; avatar?: string; attachments?: MessageAttachment[] }
+	| { channel: string | string[]; text?: string; alias?: string; emoji?: string; avatar?: string; attachments?: MessageAttachment[] };
 
 const ChatPostMessageSchema = {
 	oneOf: [
@@ -706,7 +706,15 @@ const ChatPostMessageSchema = {
 			type: 'object',
 			properties: {
 				roomId: {
-					type: 'string',
+					oneOf: [
+						{ type: 'string' },
+						{
+							type: 'array',
+							items: {
+								type: 'string',
+							},
+						},
+					],
 				},
 				text: {
 					type: 'string',
@@ -739,7 +747,15 @@ const ChatPostMessageSchema = {
 			type: 'object',
 			properties: {
 				channel: {
-					type: 'string',
+					oneOf: [
+						{ type: 'string' },
+						{
+							type: 'array',
+							items: {
+								type: 'string',
+							},
+						},
+					],
 				},
 				text: {
 					type: 'string',