How to you add a subject and predicate to: in the window seat on the plane.

The boy sat. That could be a subject. The predicate is the half-sentence you have right now. So, it could be The boy sat in the window seat on the plane.

Explanation:

A subject contains the noun. A predicate contains what the noun does.

Not so sure but could be:

She made a big fuss about not sitting in the window seat on the plane.

Explanation:

She is the subject, as it the noun and the predicate is the rest of it.