You need this cable. Google "3.5mm optical to toslink" for supplier somewhere near/ships to you.
(Minimise adapters as it merely adds potential points of failure.)
https://www.jaycar.com.au/toslink-to-3-5mm-fibre-optic-audio-lead-1-0m/p/WQ7298
Also SPDIF is a 'protocol' for information transfer...