..and the toolbar button variant (you'll have to tweak it to make it look decent):

Code:
    
    Object oContainer is a Container3d
      Set Location to 146 221
      Set Size to 15 50
      Set Border_Style to Border_ClientEdge
      
      Object cLocalCommandBar is a cCJCommandBarSystem
        Object oToolbar is a cCJToolbar
          Set pbGripper to False
          Set pbEnableDocking to False
          Set pbCloseable to False
          Set pbHideWrap to True
      
          Object oButton1 is a cCJMenuItem
            Set psCaption to 'Set Default'
            Set psImage to "XML.ico"
        
            Procedure OnExecute Variant vCommandBarControl
              Boolean bChanged bIsDeo
              Integer iFile iField iType
              Handle  hoFocus
              String  sValue
              
              Get Focus to hoFocus
              If (hoFocus) Begin
              Get is_Function Get_DEO_Control_Object hoFocus False to bIsDeo
              If (bIsDeo) Begin
                Get Data_File of hoFocus  to iFile
                Get Data_Field of hoFocus to iField
               
                Get File_Field_Current_Value of (Main_DD(Self)) iFile iField to sValue
                Get File_Field_Changed_State of (Main_DD(Self)) iFile iField to bChanged
                Get_Attribute DF_FIELD_TYPE of iFile iField to iType
                If (iType=DF_ASCII) Begin
                  Set File_Field_Changed_Value of (Main_DD(Self)) iFile iField to "NewDefault"  
                End
              End
              End
            End_Procedure
          End_Object
        End_Object
      End_Object
    End_Object