Where do US sellers put the IOSS # on the customs forms for packages going to EU countries?

This only applies to US sellers that are shipping to an EU country. 

United States booksellers will want to put that IOSS # in the"Importers Reference" fields. 

Please note: 
This does not apply for consolidated shipping (Asendia) or via DHL.

https://faq.usps.com/s/article/Import-One-Stop-Shop-IOSS