Constraint Call in BC script

Hi 

I have a already a prepaid asset, customer wants to change it to postpaid for that 

we have raised a modiy order and based on subaction of pretopost we are adding postpaid product through BC:PreSetfield value script Order Entry - Line Items.So As soon as the user select subaction as pretopost postpaid product is added to the second line item.

Now what we want to fire constraints as we want to set the attribute value of postpaid product based on the BA Credit Amount.

So please guide how to fire constraints through script in Order Entry - Line Items BC.