SELECTION-SCREEN FIELD VALIDATION

Hi All!

I am trying to write this ABAP program in 4.7 Enterprise. I wish to selectively display (based on radio buttons) selection screen blocks and make them mandatory in runtime. I use code like this:

AT SELECTION-SCREEN ON SAL_ORG. (this is one of the fields that will be obligatory)

IF RD2 EQ 'X'.

IF SAL_ORG IS INITIAL.

MESSAGE 'THIS FIELD IS MANDATORY' TYPE 'I'.

ENDIF.

ENDIF.

Now, if I execute this as is, then I will get this message as soon as the block is displayed, not just after hitting the execution button. Can you please tell me how I should put a condition on this checking. I know it is triggered by SY-UCOMM = 'ONLI' but i am not sure where to put the condition.

Thanks in advance!