Is there an easy way to get customerId ?

Is there an easy way to get customerId ?

I'm building an application for non tech team. I need to register on my side the customerId (for ZohoInvoice).
Where the end user can find the customerId of a contact ?
 (as simple as possible. Eventually an url ending by ...&customerId=1234 or invoice.zoho.com/user/1234 is ok)