Saw this old thread. Above it mentions:
The client_id is the same in all functions, it is the unique ID that identifies a client and has to be used in all functions. There is a function to get the client_id by sys_userid of a record. All functions are described in detail in the new api docs.
OK, how can one get the sys_userid associated with a VAT or a CustomerNo or any other field in the "clients" table?
Where / how is one supposed to get the sys_userid with the API?