Tabstrip on Selection Screen

TABLES: sscrfields.
* SUBSCREEN 1 For Tabstrip BUTTON1
SELECTION-SCREEN BEGIN OF SCREEN 100 AS SUBSCREEN.
SELECTION-SCREEN BEGIN OF BLOCK b1 WITH FRAME.
PARAMETERS: p1(10) TYPE c,
p2(10) TYPE c,
p3(10) TYPE c.
SELECTION-SCREEN END OF BLOCK b1.
SELECTION-SCREEN END OF SCREEN 100.
* SUBSCREEN 2 For Tabstrip BUTTON2
SELECTION-SCREEN BEGIN OF SCREEN 200 AS SUBSCREEN.
SELECTION-SCREEN BEGIN OF BLOCK b2 WITH FRAME.
PARAMETERS: q1(10) TYPE c OBLIGATORY,
q2(10) TYPE c.
SELECTION-SCREEN END OF BLOCK b2.
SELECTION-SCREEN END OF SCREEN 200.
* Go to tcode SE51 Create screen no 0300
* with screen type = subscreen
* STANDARD SELECTION SCREEN
PARAMETERS: h1(10) TYPE c.
SELECTION-SCREEN: BEGIN OF TABBED BLOCK mytab FOR 10 LINES,
TAB (20) button1 USER-COMMAND push1
DEFAULT SCREEN 100,
TAB (20) button2 USER-COMMAND push2
DEFAULT SCREEN 200,
TAB (20) button3 USER-COMMAND push3
DEFAULT SCREEN 300,
END OF BLOCK mytab.
INITIALIZATION.
button1 = 'This is sel-screen 100'.
button2 = 'This is sel-screen 200'.
button3 = 'This is screen 300'.
mytab-prog = sy-repid.
mytab-dynnr = 100.
mytab-activetab = 'BUTTON1'.
AT SELECTION-SCREEN.
CASE sy-dynnr.
WHEN 1000.
WHEN 100.
IF p1 IS INITIAL.
sscrfields-ucomm = 'PUSH1'.
MESSAGE I888(sabapdocu) WITH 'Make an entry in P1'.
ENDIF.
WHEN 200.
MESSAGE s888(sabapdocu) WITH 'Validation on' sy-dynnr.
ENDCASE.

START-OF-SELECTION.
WRITE: / 'P1:', p1,'Q1:', q1,
/ 'P2:', p2,'Q2:', q2,
/ 'P3:', p3.