set a date when an enumeration value is selected

In RTC v4.0.6, there is a work item having two fields:
a field called 'Priority reviewed' and this is a yes/no enumeration and the default value is 'no'
the other field is 'Priority reviewed date' and it's a time stamp

I want the 'Priority reviewed date' to be set automatically when the 'Priority reviewed' field is changed from 'no' to 'yes'

so how can I do this please? I tried to do this through 'attributes customization' configuration tab into RTC eclipse client but I couldn't.

Thanks in advance