Unlike searching using the user name, the search using the input field Photographer uses the photographer's real name, which is given in the picture in the IPTC field author.
Thus, for example, the pictures by the photographer "Horst Müller" could be loaded by two agencies with the user name "AgencyABC" and "AgencyDEF", plus by Horst Müller himself, under the user name "hmueller". If the photographer has entered his name in the IPTC field Author on all pictures, you can search for his pictures across all providers. To bring up the pictures by Horst Müller held by provider AgencyABC, you would have to enter as user name AgencyABC and in the field Photographer the name of the photographer.
Unfortunately, photographers frequently use different variations of their name. In the above case, you might, if the worst came to the worst, come across the forms "Horst Mueller", "H. Müller", "H. Mueller", "Müller" and "Mueller". Unfortunately, the search machine cannot decide whether all these names belong to the same photographer. So, on principle, photographers should enter their full name in the IPTC field Author and keep this form for all their pictures.