There is no way to appear in the primary inbox unless the user moves you themselves.
I'm not sure the algorithm that Google uses to classify promos vs personal mail. Maybe it recognizes multiple Gmail users getting the same message, maybe it is the email address being recognized as a company or bulk sending address. However it is, it is similar to loading images in many clients. The recipient needs to enable your images to load by default by adding you as a safe sender. Similar concept, they need to set you to their primary box for you to gain that privilege.