If you don't want the count-in to play just double click on bar 1 in Biab and it will play from there, but the count-in is hard wired in for count-ins or pickup notes as you can see in the audio edit or piano roll. There is also an option to turn it off in Prefs> Count-In/Metro.
See in the pic FL starts on bar 1 when Biab gets to bar 1. If you set FL to 0 then FL bar 1 will = Biab bar -1, FL bar 2 will = Biab bar 0, FL bar 3 will = Biab bar 1.
In most DAWs there is a bar offset adjustment but I can't see in FL but the send beat offset works.

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