As a self-publisher, your publisher like Amazon’s KDP or NotionPress isn’t the publishers of the book. They are merely a printer for the book. You are the owner of the copyright of the book. So use the word “Self Publish” as a publisher.
I used Amazon as my publisher and the application was rejected with a note:
Please include printer detail on Title-Verso page and ISBN in this category are allotted to self-published books but it is not the case with your application according to Title-verso. Please provide the title verso page mentioning publisher as self-published or name of Author.