Auto Increment field - SugarServe OnDemand

I've been trying to implement an auto-incrementing field as per https://support.sugarcrm.com/Documentation/Sugar_Developer/Sugar_Developer_Guide_10.2/Cookbook/Creating_an_Auto-Incrementing_Field/index.html

Following the guidance I've tried to deploy the class and method that the logic hook will call, but I cannot deploy to the folder custom/modules/Accounts/Accounts_Save.php - the module loader returns an error when i try to install the package.

Does anyone know the correct path to install this to when using OnDemand?