How to Default / Validate the entries in PA30 or PA40 Transaction

Use the User EXIT..

EXIT_SAPFP50M_001 - PBO
EXIT_SAPFP50M_002 - PAI

Based upon the requirement, we need to write the logic either in PAI or PBO.
Use the below logic to fetch the screen data..

DATA          : lx_0267  TYPE p0267.
Constant     : lc_0267   TYPE infty  value '0267'.    

* Check the Infotypes...
CLEAR lx_0267.
IF sy-tcode NE 'PA20' AND ( ipsyst-ioper EQ 'INS' OR ipsyst-ioper EQ 'MOD' ).
   CASE innnn-infty.
     WHEN lc_0267.
*** Importing data from innnn to the structure lx_0267
       CALL METHOD cl_hr_pnnnn_type_cast=>prelp_to_pnnnn
         EXPORTING
           prelp = innnn
         IMPORTING
           pnnnn = lx_0267.
     

Here you can write the logic... Like... Validations or pass default values..

           CALL METHOD cl_hr_pnnnn_type_cast=>pnnnn_to_prelp
             EXPORTING
               pnnnn = lx_0267
             IMPORTING
               prelp = innnn.
         WHEN OTHERS.
       ENDCASE.
     WHEN OTHERS.
   ENDCASE.
ENDIF.