Combox index

Question

Hi I have a combo box and im trying to make the index  a default string.

so for example


i want the box to formatted in this order.

Choose fruit    
 ---------------    so I want this line to be blank at all times
apple
orange
grapes
pear

after I choose and item , revert back to reset the index back to the default string, but remember i want the second line to be blank