KB21n validation to avoid cross company postings

Hi all, I would like to ask a question concerning validation. I try to install one to avoid cross company code postings with KB21n. I have found a very godd solution here for our problem but we do not have the mentioned fields in our selection within validation configuration and I don´t know how to get those fields into the selection. I need to use COBL_SENDER-BUKRS = COBL_RECEIVER-BUKRS Can somebody tell me how to get those fields into the validation structure? Thanks in advance for your help, best regards Nadine