The WiiM Pro and ProPlus models have both an Optical and digital Coax output. Use the Coax output into the Hegel's coaxial connection. When you go through the initial setup of the WiiM, if I remember correctly, it will ask you to pick the output you want to use. If you don't already have a digital coax cable, you will need to purchase one. You can buy a very cheap no name one on Amazon just to get you up and going, and then, after doing some research, etc. pick a better one from a respectable manufacturer. (And there are a lot of those ranging from $75 into the stratosphere.)
If you want to use USB instead, you would need to go to the WiiM Ultra. But that isn't necessary for what you need. And the Ultra is double the price.
-Jeff