Your auditions can either be acapella, or with background music edited in/heard in the recording. It doesn't matter what song you sing, although preferably a BTS song so I can see how well you are with their songs. You don't have to sing a full song, either- just at least 30 seconds to 1 minute of you singing in either English or Korean.

It would be even better if you could sing multiple small parts from different songs, that the member you are auditioning for sings.