Ok, I'll try to not leave anything out, but forgive me if I miss a step, trying to do this quickly. I presume you have the correct Focusrite driver installed for whichever generation Scarlett Solo you are using.
Look over these screenshots and see if you have it setup like this. Note that your driver may not be called Focusrite USB 2.0, mine is for my interface.
In the Studio tab, click on Devices.
Select ASIO as the driver type (I do not recall if ASIO needs installed separately or not, but we can figure that out if so)
After selecting ASIO, click on "Select Devices". Once that window opens, under "Audio Devices" select your Focusrite driver which will be something like "Focusrite Solo USB 2.0 Driver". It seems they have a few generations of the Scarlett SOLO, so I don't know which specific driver it will use, nonetheless, it will be something of the sort.
After that your interface should be ready, should be able to record and hear playback audio.
To set a track to record "audio" from a keyboard:
Click Add Track button, then select "Audio Track".
Now in "Track 1", click the "Rec" button, it should turn red.
Next click the smaller arrow on the right side of the "Rec" button to open the input selector drop down menu. Now click on the second input, which may be listed as "Line 1", Line 2, MIC 2, etc. I'm not sure what it will be on the Scarlett Solo, but the front panel shows that 1/4" input as number 2 input.
Note, while doing these steps, make sure your keyboard is plugged into the 1/4" input, and press keys while watching the bar graph meter in MultitrackStudio. Also take note that if you're pressing keys on the keyboard, you can see which input is active in the "drop down list " next to the record button, then you can visually see what input is active and receiving signal.
You can see the meters going in this photo to show it is getting an input signal, on mine it is on 5: Loop 1 (5) input. The green bar shows that particular input is receiving a signal. Your Scarlett SOLO should have less inputs than my 8i6 shown here, but otherwise should function the same.
Don't forget to click on the MON button to hear what you're playing out of your monitor speakers that may be connected to the Scarlett SOLO's line outputs, of course there will be some latency here. Using the Direct monitor function on your interface can make this easier to deal with.
Hopefully I didn't mess up on any of that, but perhaps will move you in the right direction. The Scarlett SOLO should work perfectly fine in MTS! I've played with a ton of different interfaces, and never had any issues.