Include Production Orders in LTP (Long Term Planning)

Experts,

if I set up a LTP Scenario (MS 31-33) and include as Receipt

Include firm recpts

Include firm purchase requisition

Include firm planned orders

our system does not take Production Orders into account.

What needs to be done to customize SAP in that way that it takes Production Orders into account for Long Term Planning. What is the applicable SPRO section.

Thanks in Advance

Jörg