You can use custom field in naming series along with YY MM DD option

I have a custom field (type: Data) under Supplier called Supplier Code → supplier_code

In naming series I tested PO-.YY.-.{supplier_code}.-.##### but that gives:
Special Characters except “-”, “#”, “.” and “/” not allowed in naming series

If I do it without the curly-brackets supplier_code doesn’t get replaced, instead gets printed as the work ‘supplier_code’.

tips?