Set default values to ITEM FIELDS in ME51N trx !

Hello guys .

I was trying to  set default values to specific fields on an item line   in trx ME51N .
Once i enter the ME51N transaction and choose the EXTERNAL PURCHASE  
I want that several  fields in the ITEM  table  to be filled up with some default values .
Trying  to use the BADI ME_PROCESS_REQ_CUST and the interface  IF_EX_ME_PROCESS_REQ_CUST

but still not sure which of the methods to use  .
I have tried already  the method PROCESS_ITEM but the default values  are set  on the ITEM LINE once i enter a value in an
item field and hit enter .
I want this  values to be Default without having  to insert any value on any Item field and then hit enter , but  directly after
i choose the type of Purchase document : External Purchase .

Any Idea ????

Thanks in advance  .