In a code language, if SUPPLY is written as QVTZMQ, then how will FORCES be written in the same language?

Let's inspect the pattern of transformation from SUPPLY to QVTZMQ by splitting the word into halves: SUP and PLY.
Let's check cross/reverse operations within segments:

S→T (+1)

U→V (+1)

P→Q (+1)
If written in cross order for the first three letters (SUP): P+1=Q, U+1=V, S+1=T. This gives QVT.

Now look at the next three letters (PLY):

P→Q (+1)

L→M (+1)

Y→Z (+1)
Reversing the order of PLY after adding +1: Y+1=Z, L+1=M, P+1=Q. This gives ZMQ.

Combined together, QVT and ZMQ form QVTZMQ.

Let's apply this exact strategy to FORCES:
First half: FOR

R+1=S

O+1=P

F+1=G
This gives SPG.

Second half: CES

S+1=T

E+1=F

C+1=D
This gives TFD.

Combining both halves yields SPGTFD.