Hilary, since we did this via e-mail, I adding your comments here for the benefit of others:
HS: Instead, I’d put a rule on the ID field in the secondary
data source, because you are probably not using that list ID for anything. You
know it will always be greater than 0, so if you set it to 0, it will change
and the rule will fire. Since this data connection is running on load, you could
simply add a rule to the FormLoad rules to set the ID in the Agency_Billing
data source to 0. Then if the rule to set the URL field is on the ID field, it
will fire and the field will be set.
DAP: Solution worked...you are not only a gentlewoman, but a scholar. Thank you.