If you have not yet created a job but have the related Supplier Invoice, you can create a Pre-Job Supplier’s Invoice. This is accessed via menu: Customs Clearance/Tariff Class > Pre-Job Supplier Invoice
By clicking the 
button or typing A will place the cursor in the input screen in Add Mode. See below:
Once a job is created for the relevant customer, it is then possible to perform a zoom on the Invoice Number field in the Supplier Invoice Screen to look up the Pre Job Supplier Invoice.