Using Set_item_property default_where

I am trying to set the where property on a Block query using the
set_item_property.
When the  user enters  a letter I wan to show a list of names beginning with that letter. I know the syntax should be for example (last_name like  'M%').

WHen the user enters the letter all the user  gets is last_name the match the letter.
 
Please tell how to set the snytax for the set_item_property  default_where clause.

my current syntax is
set_item_property(Block_id,default_where, 'Last_name like :Block.last_name');
where :Block.last_name being the field where the user enters the letter.