Aspect ratio: how to prevent Canon Selphy CP1300 from trimming my photos without too big an edge

My smartphone takes pictures with a different ratio than the Canon Selphy CP1300 printer (10×15 cm).

When I print the photo using the "Canon PRINT Inkjet / SELPHY" application using the default "Borderless" setting, the printer cuts the photo vertically. When I set it to "Bordered", the printer adds a large border both horizontally and vertically.

How can I tell the printer to print the photo as large as possible without cutting it (that is, with a border of zero height and a minimum horizontal border)?

PS: If possible, without using a photo editing software to add a fictional horizontal border to get the same ratio as the printer.