Thank you! I found it.  It works now.

Thanks for your answer. I tried to use the Ctrl+Option+Cmd+D to put a file into Dropzone from Finder but nothing happened.  Do I have to change something in the Dropzone settings?

I have a (naïve) question.  I know that it is possible to send a file to Dropzone by pressing F3 and write the corresponding shorcut letter, but... is it possible to drag and drop the files from dropzone to the active window with the keyboard without using the mouse or trackpad?