What is the prefix for dessert?

There isn't a specific prefix for "dessert" in the same way there are for words like "pre-" or "post-".

However, you could use "post-" to indicate something that comes after the main meal, as in "post-dinner dessert".

Or, you could use "sweet-" to indicate the sweet nature of a dessert, as in "sweet treat".

Ultimately, the best word to use depends on the specific context of your sentence.