1. Register for Uptime Robot API key @ https://uptimerobot.com/signUp
  2. Go to Uptime Robot account > my settings
  3. Look for API settings > main API key
  4. Click on create the main API key then click on create the main API key button  
  5. Copy the API key then go to WHMCS admin area
  6. Navigate to ADDONS > Uptime Monitor > Settings
  7. Enter you API key and click save changes