Help with an IIF query required

Hi,

How would I write this IIF query into an Outbound Sync Rule:

If Boolean_attribute_X is False, export the following 'static' value into the target_AD_attribute,

else do nothing (if true)

My thinking is something along these lines for the custom expression:

IIF(Boolean_Attribute, Customexpression(NULL())), "Static Value"

Thank you