User-exit for VL02N

Dear ALL,

Could you please provide me user-exit for VL02n? My requirement is when i enter tcode VL02n a pop up screen must come asking for certificate number to enter for the line item materials in delivery. After entering the certificate number it must enter VL02n and list all the line item of delivery.

I can find user-exit after entering VL02n and after making some changes in VL02n or before saving delivery or at the time of PGI. But i need when we type VL02n and enter.

Badi or user-exit which ever might be fine. Pls let me know if some one knows exit at the time of entering VL02n.

Thanks for your help.

Regards,

Sathish