Login to the Toast portal
Go to your menu
Click on a menu item
Look at the URL in your browser address bar and you will see the GUID
If the menu item is used in multiple menus, you will need to append the menu GUID (see example below)
Example GUID
4ab87566-6586-4f50-9030-644ea06dd4c5
Example GUID with appended Menu GUID
4ab87566-6586-4f50-9030-644ea06dd4c5>menu=10befce7-7ec4-4363-8e5e-1379b770bc65