See Section 2: Gather Your Resources for more information about ISBNs.

A screenshot from the KDP publishing process. The heading is 'ISBN' and there are two options.

Option 1: Get a free KDP ISBN.
Option 2: Add your own ISBN.

Imprint – If you have registered your ISBN with an imprint outside of KDP, enter the imprint name exactly as it appears in your ISBN registration. If you are using the ISBN provided by Amazon, you can leave this section blank.

A screenshot from the KDP publishing process. The heading is 'ISBN' and the second option, 'Add your own ISBN' has been selected. 

There are spaces to input the ISBN and the publisher.