How to add new photos in slideshow that appear in order at the end / back of previously added photos

Hi - I'm creating my first slideshow in Mac Photos --> Slideshow. The intent of the slideshow is to have photos in chronological order of my daughter's life (we will play the show for a big event for her). I start by adding a few photos when she was a baby. But every time I add new photos, it puts them at the START of the slideshow order, rather than the end (after what has been previously added to the slide show). For every new photo (or batch of photos) that I add to the slideshow, I want them to appear in order at the back / AFTER what I have previously added...but instead it always adds them at the start / front. This seems totally backwards and wrong. New added photos should appear after what has already been added. How can this be done? Is there a setting that will allow this, and where is that setting? I fully realize that you can drag-and-drop the order of photos once they have been added, but that does not specifically address what I am trying to accomplish. Please help! Thank you.