So you want to forward an older transfer but add some new files too? You have probably noticed that this is not currently possible as you can only choose to send a new transfer or to forward an old transfer from your transfer overview but not both options together.
This is where this trick comes in handy!
If you add your files normally as you usually do to make a transfer and then copy the download link from your older transfer in the transfer overview, you can then paste the download link in the message section of the transfer window.
This way, the recipient will receive the older files and the newer files you have uploaded all together in one nice package!
Bear in mind you will only receive a download confirmation for the new files uploaded and not for the added link.