A couple of strategically placed cup hooks?
M-F extension cable as @PaulThePlug suggests, held in place with a P-clip (eg https://www.ebay.co.uk/itm/171504770946 or https://www.ebay.co.uk/itm/322547600814) or U-clamp (eg https://www.ebay.co.uk/itm/394456647518). Or, if you can solder, put a couple of stereo jacks into a small Hammond box (1590A) and connect T-T, R-R, S-S, then fix it to the underside of the desk.