Hello Sadiq,
A simple fix I have used in the past is to set Reason Code 0000 to 'No Show' - it will default in unless the learner/admin changes it. But it requires people to pay attention and understand that they don't want No Show to appear unless it is the valid reason. The other option you can explore is BAdI LSO_CANCELREASONS_C It may provide the functionality you need - I have only used it to provide valid drop-down reasons in the portal for Learners. Good luck, Sharon