I have used your code of valid combo method, but when I click on the list box appear the message "This can only be used within a method".

Then you did not use my code in the Valid method of your combo or else you would not have gotten this error. Look more closely at the code (modified for use with numeric value) in the Valid method of my combo:

PROCEDURE combo1.Valid
  LOCAL lcFilter
  SELECT < Table used to populate RowSource of List Box >
  lcFilter = [ < field in List Box Cursor that defines the filter > ] + TRANSFORM( this.Value  )
  SET FILTER TO &lcFilter
  GO TOP
  Thisform.List1.Requery()
ENDPROC

I am guessing that the reason you are getting the error you describe is that you are trying to set the filter using this (incorrect) syntax:

SET FILTER TO < Field in List Box Cursor > = This.Value