Hi Hubert,
generally you can find all the dynpro numbers in transaction SE80 when using function group CV130. Here in the folder "Screens" all objects are listed.
For purchase requisition issues I would recommend you to maintain at least objects EBAN and EKPO. In note 375452 you will find the necessary screen numbers. The entries can be made also via the DMS customizing in transaction SPRO at
Cross-Application Components
>> Document Management
>> Control Data
>> Main Screen for object link
Best regards,
Christoph