The backing track, assuming it is a wav file, is going to be put into the BB drum kit as the “drum sample” for the kick drum at 36, if you follow my example.
In Logic, you are going to add the wav file to a blank audio track. The ONLY reason you are doing this is to get it’s length. Then, on a separate software instrument (midi) track, your are going to create a long midi note at C1 (midi 36), which will be as long as the audio track. This software instrument track then gets exported as a midi file. (select just that track, then File>Export>Selection as Midi File.)
Then you create the BB Song. Add the midi file to the song as the intro. Select the dedicated drum kit that you created in step 1. Triggering that single midi note, when you press the pedal to play the song on the BB, will play the backing track.
One additional note - I have had issues with Logic adding meta-tags to wav files created in Logic. These meta-tags somehow conflict with BB manager and cause the wav file to be unplayable. I use a tag stripper program to remove the tags before placing the wav file in the drum kit. If your wav file was not created in Logic, this should not present a problem. But, if it does, the program I use is Switch from NCH Software. I simply add the wav file to Switch and then save it as a wav file once more. Verify that the file is set to save as at 16bit, 44.1khz wav.