Validation logic in custom infotype

Hi Gurus,

I have created a custom PA infotype.

I need to add a validation logic on 'BEGDA' and 'ENDDA' which should get triggered, when I try to save the infotype.

The logic is very small one., but am not able to understand where to add the validation logic.

I have written the validation logic in the related module pool program of screen 2000 in PAI module., but its not getting triggered.

Below is the screen 2000 flow logic.

Kindly let me know where to write the logic.

PROCESS　BEFORE OUTPUT.

MODULE BEFORE_OUTPUT.
CALL SUBSCREEN　subscreen_empl　　　INCLUDING　empl_prog　empl_dynnr.
CALL SUBSCREEN　subscreen_header　INCLUDING　header_prog　header_dynnr.
MODULE P9123.
MODULE HIDDEN_DATA.


PROCESS　AFTER INPUT.
MODULE EXIT AT EXIT-COMMAND.
CHAIN.
FIELD P9123-BEGDA.
FIELD P9123-ENDDA.
FIELD P9123-ZABC.
MODULE INPUT_STATUS ON CHAIN-REQUEST.
ENDCHAIN.


MODULE PRE_INPUT_CHECKS.
CHAIN.
FIELD P9123-BEGDA.
FIELD P9123-ENDDA.
FIELD RP50M-SPRTX.
FIELD P9123-ZABC.
MODULE POST_INPUT_CHECKS.
ENDCHAIN.

Thanks in advance,

Goutham.