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*.