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There is no such thing as technique, only practice, lots of practice, practicing at home on easy pieces, and then improving the experience gained on slightly less easy pieces.

If you have studied or are studying harmony, the task will be much easier.

My harmony teacher used to make me play the harmonization examples on the piano in all keys.

After many years, I still practice, especially when I accompany polyphonic choirs or simple single-voice chants on the organ.

I realize that if you have only been studying piano for a few years, the task will not be easy.

Study harmony and always transpose - with consistency - and you will become veryyyyyy good.

Here are some useful ideas, there are methods... even progressive ones like Norris or Fiorino, but a lot of practice is needed.

www.pianoconcerto.it/forum/index.php?/topic/1009-migliorare-nella-lettura-a-prima-vista-e-trasporto

Thanks to both of you for the answers!

As always happens with music, practice, study and exercise are the right solution for every difficulty!

If there is no mandatory written accompaniment (perhaps it's not original and is a transcription anyway), you can try to identify the harmony and derive the chords to support the choir. Using transposition rules is difficult. It might work well on a sung melodic line, but instrumentally it is extremely difficult. However, if it involves a chromatic semitone below or above, it is easier because you don't have to read in a new key (you think of or write the accidentals of the new key and raise or lower the temporary accidentals you encounter along the way—perhaps you can even note them down).