BADI Implementation

Hi all,

In Transaction ME21N, there is a field 'OUR REFERENCE' in the Communication Tab. Now, we need to track the value entered and perform some ztable updation. I couldnt find a user-exit for the same but someone told me to implement a BADI ME_PROCESS_PO_CUST. I created a new BADI in SE19 and put the BADI name in the definition. i went inside one of the methods ( POST ) and tried putting the following code there.

if MEPOCOMM-UNSEZ is initial.

**************

endif.

but the code is giving syntax error saying the field MEPOCOMM-UNSEZ is unknown. so its not getting activated. the field is the screen field name of the 'Our Reference' in ME21N.

is there something wrong with the interface?? is there somethng to be specified in Import Parameters or something?? I am new to BADIs. So please explain in a simple manner. Also, how to search for the BADI definition for a particular purpose in the first place?? I would like to know the procedure for how we go about finding the suitable BADI and then implementing it. I have already read the previous threads regarding BADIs.

All useful Inputs would be appropriately Acknowledged,

Thanks & Regards,

Bikash