Implementing sequential voucher numbering

Good morning

We are trying to implement automatic sequential voucher numbering in Payables and are having a few problems. We have enabled the option in the systems profile values window. We have also defined document category and sequence and then assigned the sequence to the category. We have also enabled the document category override options in the payables options window.

What we dont know how to do is how we then make the voucher number field pick up these settings. When we have tried to enter a new invoice batch the voucher number field is still a manual entry field.