In order to create your Schaeffler related account on Tradeshift you must accept the invitation to join Tradeshift that was or will be sent from Tradeshift on behalf of Schaeffler. Please consider your “spam” inbox as well in case you expected such an invitation but have not yet received it.
When the connection is activated you will be able to invoice Schaeffler via the methods described above. 
Further details can be found here.