Hooks for category pages

Hi there,

I want to add a dropdown for my category pages but cannot find the right hook for what I am requesting. if I use before main content, it appears as this (where the first article appears to the right of it).

If I choose to do it as after header, it appears correctly, but the sidebar is pushed down which I do not want.

Is there a hook/way to achieve the desired look I am after with the hooks of GP?

Many thanks,

Will