Open ''Photos On Mac'' with short command directly to other location/External Harddrive

I remember it was a ''short command'' that could run an application like ''Photos On Mac'' which made it possible to change to what location/external hard drive the app directly would run. It was as easy as close the ''Photos On Mac'' - make this ''short command'' that I have forgotten - and a window pops up which made it possible for me to run the application directly into my hard drive. The reason is that my computer is full of storage and doing this I could then directly import the pictures to my hard drive. Do anyone knows this ''short command?'' - Thanks in advance!