Digital output from Raspi to internal DAC of amplifier

I would like to replace my Chromecast Audio with a Raspi 3B+ and Volumio. My music files are on a NAS and I own an Onkyo TX-8050 including an internal DAC (optical input). So I don’t need a DAC like Hifiberry.
My question: Is there a way to transfer the digital signal from my Raspi directly to the optical input of my Onkyo? Do I need additional hardware?
I am a rookie, any help would be appreciated!

You need something like a Digi+

But I strongly recomend to get a digital output boad that offers oscillators for both smapling frequency families like Kali Digione and use the S/PDIF output

The Clock Recovery by PLL used in the Wolfson chip of the digi+ has poor jitter performance.