Horizontally(unless you're holding the bird on it's side :D )....
....here's a diagram, I measure between F-F:
https://www.backyardchickens.com/articles/who-is-laying-and-who-is-not-butt-check.73309/
She may already be laying.
Do you free range?
Free range birds sometimes need to be 'trained'(or re-trained) to lay in the coop nests, especially new layers. Leaving them locked in the coop/run for a week or so can help 'home' them to lay in the coop nests. Fake eggs/golf balls in the nests can...