help for a choise box

Hello g00r00 and thanks for all your replies, please, just another one :-)))
I created the private messages menu and inside there is the choice "write
message". I would like a box with 2 items to open: emails and netmails, I think 
it should be done with a generic prompt of choosing a or b but I don't know how 
to do it, could you help me please?