getting the purchase req number in APO

Hi Experts,

We have activated the APO check. when we create the sales order in ECC GATP check happens in SCM and the purchase req is generated in ECC and the number is transferred to SCM. which i can see in the RRP3 tcode in SCM. Now i want to capture this purchase req number which is generated in ECC and going to SCM. i have tried lot of user exits but could not succeed. is there any user exit /badi in ECC or in SCM where we can capture the purchase req number that is going to SCM.

thanks