In FileDrop go to the settings tab and click on the folder icon.
This feature is available only for certain paid plans.
Here click on the folder icon or text and the Drive folder selection modal will open.
The open folder link will open the current folder where the files are saved.
Here you can select any folder you want from your Drive, Shared Drives or folders shared by other users.
1. First select the folder, you can browse to any folder you wish.
2. Click the select button to save the selection.
You might need to restart FileDrop for the change to be reflected on Settings tab.