The is the tune from Bach ‘O Herzenasangst’; Riemenschneider No 173
My harmonisation attempt adding A, T, B in open score:
As a WAV file generated by Sibelius:
Looking now at the Bach original, he harmonises the ‘-angst’ in a way I would not have guessed, as the third inversion of a dominant seventh, resolving upwards with a jump upwards of a major seventh in the bass line!
WAV file of Bach harmonisation