How do I represent a Bb(b9)13 chord in chordmode?

[Carl D. Sorensen]

Try

bes:13.9-

bes:13 gives you the Bb13; .9- gives you the flatted 9.  Of course, this
chord will be missing the 11.  If you want the 11 added, you'd do
bes:13.11.9-

HTH,

Carl