The instructions in the (German) user manual imply, that the printer was first properly attached to the computer itself. The user manual explains, how to change the settings of the printer configuration, so that you can make use of that printer, which is now attached to the FRITZ!Box.
You can also install it as a network printer right away, but be patient and trust in the Windows set up wizzard.
Let it retrieve the complete list of printers from the Internet, in case your printers are as ancient as mine!
You won’t need to tell it, it’s port 9100, the Windows installation procedure takes just as long as it announced: several minutes.