Hello Enrique,
first check how the BAPI function module processes that parameter.
The function module calls a subroutine PR_EXTENSIONIN in line 224.
If you follow the subroutine, you see starting with line 1088 what it does in your case when the structure field of EXTENSIONIN is set to 'BAPI_TE_REQUISITION_ITEM'.
To me it looks as though you only put the Item Number of Purchase Requisition of type bapi_te_requisition_item into the EXTENSIONIN-VALUEPART1 for your case.
If that does not help, check how others call the BAPI function module.
Best regards,
Frank.