I'm not sure that's possible unless the helicon has a harmony output only facility. i think it has a left and right balanced xlr out but unless one of them can be set to harmony only you can't do it.

but why not create the harmony in BIAB or RB? i don't use BIAB for harmony but the manual will tell you how. RB is easy and you can set the harmonies to separate tracks which means you sing a melody line and then you can export the harmony track to your DAW.

what a lot of people do is use the harmony creation track as a guide and then record the harmony using their real voice.