If REPLY is coded as QDOKX, then STICK is coded as

Concept: In this alphabet coding-decoding pattern, every letter of the given word is replaced by the letter that sits exactly one position earlier in the English alphabet (A=1, B=2, ... Z=26) - a constant one-step backward substitution applied uniformly to every letter of the word, in the same order.

Application - confirm the rule from the given clue (REPLY -> QDOKX):

Each letter of REPLY drops by exactly one position to give Q, D, O, K, X in the same order - QDOKX - matching the clue and confirming the rule.

Now apply the same constant backward-shift-by-1 rule to STICK, letter by letter and in order:

Reading the coded letters in order gives R, S, H, B, J - RSHBJ.

Cross-check: reverse the operation by adding 1 back to every letter of RSHBJ - R+1=S, S+1=T, H+1=I, B+1=C, J+1=K - which reconstructs STICK exactly, confirming the decoding is consistent both ways.

So STICK is coded as RSHBJ.