In Boxes, power down your XP VM, click Settings -> Sharing Panel -> Enable Sharing toggle. Click File Sharing and enable File Sharing. Power on the VM.
At that point you should be able to drag and drop from your host direct into your VM for a file transfer.
You can also click the vertical dots menu in the Guest's console "screen" and click Send File... menu option.
In the same menu you can click Devices & Shares -> Realtek USB or whatever -> Local Folder -> Select from the dropdown for the Host's folder that you'd like to share -> Save -> Make sure Toggle on the right is on.
Then your folder, I believe in XP, will show up as a removable drive like a USB drive would.