Add a control/spe action to field_b and validate, that would add the "c_" subprogram section in the spe program. Then a you'll modify the c_ subprogram as below , change XXX with your screen name, change "ABC" with your condition

Subprog C_FIELD_B(VALEUR)
Variable Char VALEUR()

If VALEUR = "" and [M:XXX]FIELD_A= "ABC" Then
    GMESSAGE = "This field cannot be blank"
    mkstat=2
Endif

End

Hi Stacey Smith ZA 
I can probably do the same with regular setup using control tables (GESACL function)!

Hi Julien, yes - you're right, control tables can work but in this scenario that is being tested, not as effective. Thanks though

Thanks for this Israel, very useful and have started testing the scenarios with the script