The original manuscript is extant for the last part of the verb desire, which in 𝓞 has no pasttense d at the end of the word. This means that 𝓞 very likely read did desire (spacing between extant fragments of 𝓞 has room for the did ). In the printer’s manuscript, Oliver Cowdery accidentally wrote did desired. The 1830 compositor correctly guessed that this stood for did desire rather than desired alone. Interestingly, Oliver made a similar error in 1 Nephi 2:5, but in 𝓞 rather than in 𝓟. There he wrote did travel in 𝓞; then later (with somewhat heavier ink flow) he corrected that to did traveld in 𝓞. As explained under that passage, the original text appears to have read did travel, not traveled.
Summary: Maintain in Ether 6:21 the apparent reading of the original manuscript, namely, “they did desire of them” (not “they desired of them”).