Bb7sus4/E# (Bb 7sus4 over E#)
Notes, from the bass up
E# (bass)G#A#D#
A slash chord names two things at once. The part before the slash is the chord your right hand plays, and the part after it is the single bass note for your left hand. Read Bb7sus4/E# as Bb 7sus4 with E# in the bass.
The bass note here is one of the chord tones, so this is exactly the 2nd inversion of Bb 7sus4, written the way sheet music and chord charts usually write it.