Create an account ("invoice") in ictime. Alternatively, use the directInvoice feature in /icbiz/1.0/worklog/report.
icbiz JavaDoc
For details, refer to http://www.interconcept.de/bizdoc/de/iconcept/icbiz/rest/InvoiceRes.html.
ictime Documentation
For ictime documentation, refer to Charge Work Logs - Create Invoice.