I've been trying to get an optical signal into my Focusrite 18i8 but so far, none of the devices I've tried has worked. Most recently, I've been trying to connect the optical output of my PC's soundcard to the 18i8 audio interface but I haven't had any luck.

Is this even possible? If it is, what needs to be done to get this working?

Answer: The optical output from my PC is S/PDIF. On the other hand, my audio interface has ADAT in. The two don't speak the same language.

What is the on-board PC soundcard?
Is the optical output of this souncard active?  
 
If you put a fiber light pipe cable into it you should see a red light shining out the end to show it is active.
 

Is the digital output set as Default Audio Playback device?   May be one reason.

What make/model is your sound card?

Does it have ADAT optical out or more likely SPDIF optical out?

Works here, optical out from the motherboard; you will see it glowing red as described above.
That into the RME Babyface.
Looks like this in SONAR when selecting the optical input.

Notice I'm using "friendly" names for my I\O.
Make sure your optical input is selected in Preferences>Audio>Devices
And you have to assign your PC audio out to the optical in Control Panel (Win 7 anyway),
again as described above.
Wish you could select both there, analog and digital...
HTH

Shambler
What make/model is your sound card?

Does it have ADAT optical out or more likely SPDIF optical out?

I think I've found the problem.
As you pointed out, the optical out on my PC's soundcard is probably SPDIF. Not ADAT like on the 18i8.

Wookiee
What is the on-board PC soundcard?
Is the optical output of this souncard active?  
 
If you put a fiber light pipe cable into it you should see a red light shining out the end to show it is active.

I can see the red light at the end off the cable but I think I've figured out why it's not working... my PC's optical out is most-likely SPDIF, not ADAT like the input of my audio interface.

DeeringAmps
Works here, optical out from the motherboard; you will see it glowing red as described above.
That into the RME Babyface.
Looks like this in SONAR when selecting the optical input.

Notice I'm using "friendly" names for my I\O.
Make sure your optical input is selected in Preferences>Audio>Devices

Did you get it working?
In my case, the optical output of my PC is SPDIF, not ADAT. The optical input of my audio interface is ADAT. That's where the problem is.

Did you get it working?
It works.
I assume you are trying to record from YouTube?
You have to set Windows Sound like this

If your optical cable is "hot" (glowing red) I think it should work.
The Realtek on board audio is pretty generic, most motherboards have it; right?
You might have to look at Device Manager and be sure the Realtek is active.
Make sure your ADAT in is selected in SONAR Preferences>Audio>Devices
HTH,
T
Again, this is all in Win 7; I have no idea how M$ has organized Win 8 or 10

I think the issue is as the OP has found. There is both multi channel and stereo digital optical in/out using the same connection. The device has to support one or the other. On board sound cards have that connection to use with a Multi channel as in 5.1 or 7.1 home theatre systems.   There has always been funky ways to use it for DAW recording,,but. I would think that's not the same as ADAT. 
 
It's irrelevant anyways. You access the ADAT in/ out via the USB ( Firewire) connection to the interface using the ASIO driver. 
There's no point in using the connection from your sound card, besides, you can't use ASIO mode and to see the sound card active in Sonar. You would need to swicth to an inferior mode. In Toms Post #8  it clearly shows his Fireface USB output via the Stereo ADAT. That is not using the on board connection.