You'll need to describe the design of your database in more detail.
Assuming that there is only one client to one campaign, but possibly more than one campaign for a given client, you might have these relationships:
Clients::__pkClientID = Campaigns::_fkClientID
See Common Forum Relationship and Field Notations Explained if this notation is unfamiliar.
Then you have any number of ways to make sure that a given record in campaign gets the correct client ID. The simplest is with a portal to Campaigns placed on the Clients layout with "allow creation of records via this relationship" specified for Campaigns in the relationship. Then you can simply add data into the "add" row of the portal to create a new related record linked to that Client record.