Steps for Creating Searching Help
Screen 1.
  SE11.
  Choose Radio Button ‘Search Help’.
  Enter the name of search help starting with y/z
  Click Create.
Screen 2.
     Write Short Description.
      TAB Defination
        Enter Selection Method ‘TABLE NAME’.
        Enter Dialog Type ’Display values immediately’
        Enter Search Help Parameters Fields to which search 
        help is to be attached.
        Check Imp and exp Box
        Lpos and spos.
Screen 1.
  SE11.
  Choose Radio Button ‘Database table’.
  Enter the name of table to which the search help is to be 
attached.
  Click Change.
Screen 4.
  Check the fields in the table to which the search help is 
to be attached.(They should be  
                               the same as defined in the 
Screen 2 Search Help Parameters Fields)
  Click the Search help button .

SCREEN 3.
  Enter the search help name
  Click Generate purposal
  Click Copy.
Screen 4.
   Click on Utilities on MENU BAR
   Choose Table maintenance generator
Screen 5.
    Enter Authorization Group ‘&NC&’
    Enter Function Group
    Choose maintenance type
    Maintenance screen NO.
    Choose create.
    Save and Activate
Screen 4.
   Save and Activate.

there are some function modules which you can use on a 
field in selection screen.You have to pass the name of the 
internal table which holds the possible F4 values to this 
function module.
This function module starts from F4* somethin...i dont 
remember exactly