I use OTRS and not all clients want register in this system. In OTRS GUI I use a page (example.com/otrs/index.pl?Action=AgentTicketEmail) for create ticket by email.
Last time I begin use OTRS API for creating ticket, but I don`t understood how use this method for unregister users.
Screen the form for creating ticket by email
You're right, the TicketCreate operation mandates having an existing customer user in OTRS. You could modify the TicketCreate operation (specified in Kernel/GenericInterface/Operation/Ticket/TicketCreate.pm
) to not require this; or you could create a new operation where you could register your customers if needed.