You will need to add a link on your page. Add a new 'Content' block on your page, create a link in the editor and set URL to be the hash followed by unique id, for example #my-map. Set a custom class of the content block to 'toggle-fullscreen-map'. Then set a custom class on the map to be the same as your unique ID, so in our case it will be 'my-map' because we used it on the link previously. Save it and you wil; be able to expand a map on click of the link.