MSS leave request rejection reason mandatory

Hi to all,

we are using NWBC on our EhP6 System for ESS/MSS. I managed to configure leave request with custom workflow (just copy of the standard one with 2 extra mail steps). Now i'm trying to implement the following functionality:

The manager should not be able to reject selected leave request without typing a reason (see image below).

Which classes and BaDIs should be enhanced to implement this functionality?

Thanks in advance for the help.

Regards,

Yanko