If you have the option, it is better to use prepositions.
Zamenhof-Taga Defio ĉe Duolingo
Zamenhof-Taga Duolinga Defio
As you know, when you need to use a noun as an adjective, you must either add the -a ending or insert a hyphen. I have used Zamenhof-Taga instead of Zamenhofa Taga in order to make the structure of the expression clearer. (I think the punctuation Zamenhoftaga is more common, but this is easier to read.)
Modern English has always permitted nouns to be used as adjectives, but the compulsion to use long noun-strings or noun-sequences is fairly new, dating from World War I or so. For example, the expression "enemy forces" was not used before that time; people wrote "hostile forces" or "enemy's forces" instead.
The sudden popularity of the long noun-string is usually attributed to the influx of German-American journalists into the English-language press in 1900-1920. German scientific writing may also have contributed. It is suited to headlines and is helpful in creating certain kinds of neat technical terms (e.g. English-language press).
Nowadays the BBC routinely puts up expressions like "China summit" and "Turkey coup" even when there is no shortage of space, which is a bit unfortunate as people learn English from the BBC, and these formations sound inane and add unnecessary ambiguities. In Esperanto you would say Ĉinia kongreso and Ŝtatrenverso en Turkujo.
