calling a screen from BADI BBP_DOC_CHECK_BADI

Hi

Can anyone let me know how to call a screen from the BADI BBP_DOC_CHECK_BADI..?

We have a requirement that, when the user doesnt input in a mandatory field, after the error message pops up (we have already implemented this in the BADI) - the user should be directed to the respective page to key-in the value in the mandatary field.

Any FM's that can be used for calling in the screen inside this BADI??

We used GET parameter in this BADI and SET parameter in the BBP_CUF_BADI for calling the screen, but still unable to do so...

Any inputs, code snippets are welcome...

Rgds

Reddy