When you sign up for StreetLib, you can enter your pseudonym (if you are a self-publisher who publishes under a pseudonym) or your company name (if you're registered as a publisher) on your Account page.
This will then become the default "publisher".
You can also add other publishers if needed via Management > Publishers e.g. if you are self-publisher who wants to publish under different pseudonyms or want to distribute only some pseudonym books, or a publisher who needs to publish their books under different editorial editions.
Here is how to create a publisher and select it when publishing your book: