Designer doesn’t support optional prompts yet. They have to be created at the report’s query level. And they are available only in Web-I XI 3.x.
Alternatively in designer in all the versions, you can achieve the same by including ‘ALL’ in LOV. Please search B 
B. You’ll get enough topics. One such is this topic.
Jansi 
(BOB member since 2008-05-12)