I'm not sure, but I got it to work great on my first try with the following prompt:

------ Your task is to encode a secret word by making a sentence where each word starts with a letter in the secret word in order. For example, encoding the secret word 'bag' could produce the sentence 'bagels are green'. Encode the following secret word: 'pen' ------ People eat noodles. ------

Worked for window, backpack as well, although I did have to tell it not to use the secret word in its encoding when I got to backpack, and then to follow the same order and not repeat words after a few attempts.