Workflow connecting Related Fields

So I have two apps that I am looking to connect through a workflow. I'll try to explain this as clearly as possible. 

I have a "Marketing Campaign" app, each item within the app is a different marketing campaign. What I am looking to do is automatically link the item (the marketing campaign) to a new lead. I can create the trigger, however, I cant find a way to link the Specific marketing campaign (an item) to the related field Category within the lead. 

I can manually go into each lead, go to the related field, and choose the item which is the marketing campaign. But I would really like this to happen manually. Any advice?