Simple Mattermost Bot via Google Apps Script and Outgoing Webhook

code.gs

/*************

MATTERMOST OUTGOING WEBHOOK BOT

INSTRUCTIONS

Create a Google Spreadsheet

In column A of Sheet1, put a list of however many potential responses you'd like.

Spreadsheet menu > Tools > Script Editor
Paste in this entire file of code as the 'code.gs'

Script Editor menu > File > Project Properties > Script Properties
Add a row with key name 'verifyToken' and value of the webhook token.

Customize the trigger word below.

Script Editor menu > Publish > Deploy as web app...

Grab the URL and put that in the outgoing webhook configuration.

[OPTIONAL]
Include BetterLog (https://github.com/peterherrmann/BetterLog)
menu > Resources > Libraries...
Find a Library > project key MYB7yzedMbnJaMKECt6Sm7FLDhaBgl_dE
Choose the latest version in the dropdown > Save

**************/
Logger = BetterLog.useSpreadsheet(SpreadsheetApp.getActiveSpreadsheet().getId())

var trigger_word = "!mytrigger"

var sheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName("Sheet1")
var range = sheet.getDataRange()
var values = range.getValues()
var rows = range.getNumRows()

// respond to HTTP POST requests
function doPost(e) {
  Logger.log("e = "+JSON.stringify(e))
             
  //Convert incoming message to JSON
  var incoming = JSON.parse(e.postData.contents)
  Logger.log("incoming = "+JSON.stringify(incoming))
  
  //Get Script properties.
  var prop =  PropertiesService.getScriptProperties().getProperties()

  if (prop.verifyToken != incoming.token) {
    throw new Error("invalid token.")
  }

  if (incoming.text.toLowerCase().indexOf(trigger_word) > -1) {
    // randomly select a quote from the array pulled from the spreadsheet
    var i = Math.floor(Math.random() * rows)
    var message = values[i][0] 
  }
    
  output = JSON.stringify({"text": message})
  
  return ContentService.createTextOutput(output).setMimeType(ContentService.MimeType.JSON)
}

This works with Mattermost 5.25.0