How to debug user exit ZXF08U04 for message type PEXR2002

Hi Guru

Currntly, I am using the message PAYEXT for baiss type PEXR2002 to generate the Idoc via automatic payment program F110. As I need to map the payment method according to the requirement that provided by bank. Therefore, I have make changes in user exit ZXF08U04 with a break point set. However, program doesnt pass through User Exit .

I have ensure the user exit is activated. Can anyone share with me how can I debug the user exit?

Thank you

Kang Ring