Re: how to display dynamic text below parameters of selection screen on execution?

You can display it any where playing around selection screen statements like the comment statement and skip statement in selection screen.

http://help.sap.com/saphelp_nw70/helpdata/en/9f/dba7a135c111d1829f0000e829fbfe/content.htm

If you dont want to display it below the parameter as given above use the skip statement.

SELECTION-SCREEN beginofBLOCK bl1  WITHFRAMETITLEtext-001..
PARAMETERS: bukrs type bukrs .
PARAMETERS:           matnr type matnr.
SELECTION-SCREENendofBLOCK bl1.

SELECTION-SCREENSKIP4.
SELECTION-SCREENSKIP2.
SELECTION-SCREENbeginofBLOCK bl2  WITHFRAMETITLEtext-002.
SELECTION-SCREENCOMMENT /10(30) comm1.
SELECTION-SCREENCOMMENT /10(30) comm2.
SELECTION-SCREENendofBLOCK bl2.


AT SELECTION-SCREEN.

AT SELECTION-SCREENOUTPUT.
   ifnot bukrs isINITIAL.
    if bukrs ne'1000'and bukrs ne'2000'  .
      comm1 = 'Enter valid company code'.
    else.
    clear comm1.
endif.
endif.
   if matnr CA'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.
     comm2 = 'Enter valid material no'.
   else.
     clear comm2.
  endif.