From what I gather, it’s a noun, but is translated as a preoposition in English. It’s more like “Desk’s Underneath.”
It seems a lot of concepts are different parts of speech in English and Japanese. It appears like is usually used as a noun in Japanese and a verb in English, for example.
On the subject of 好き, I was watching Frozen in Japanese the other day and heard 彼は私を好き? And was pretty surprised that it could take the particle を. But indeed, it can.