How do you finger the lowest note on the Baritone saxophone?

Answer 1

It is the A# with the low key

the low key is the key that is below your left thumb in rest

or

The low key is the key that is below the octave key and the left thumb rest position.

Answer 2

Place your fingers as if you were playing low C and then add the low Bb key with your left pinkie. On most models of Saxophone the low Bb key is located underneath the G#, low C# and low B keys. On some models, the low Bb key is located underneath the G# key on the key closest to the body of the saxophone.

As low Bb is the lowest note, it is quite tricky to play. To get used to it, slur low F,E,D,C then low Bb. Once you get the hang of that you can try playing low Bb without running down to it.