BADI Implementation of ME_PROCESS_PO_CUST(ME22N) for error message

We are working on a BADI for transaction ME22N

Here we need to display error message if the length of header text is greater than 20.

We have explored all options and are using BADI Definition ME_PROCESS_PO_CUST for our purpose.

But here we are facing problem as when we try to display the error message it is getting displayed at item level and the item gets marked as errored.

We tried using Macro mmpur_message to display error message.

<b>We want to display the error message at header level.</b>