If using API for shortcuts, the manual reads:
A GUID can be used to specify an Input instead of a number.
For example:

http://127.0.0.1:8088/API/?Function=Fade&Duration=1000&Input=877bb3e7-58bd-46a1-85ce-0d673aec6bf5

Where do I find the GUID for each input?

Thanks Martin!
For other users who are learning about the API stuff like me and find this later, the Inputs receive a unique id (GUID) each time you add the device to vMix and if you save your layout as a Preset then the GUID is saved in the preset. So if you have 4 camera inputs and you save the Preset, when you open that preset, then each camera should have the same GUID. However, if you don't save the preset, close vmix and add the 4 cameras back to vMix, then they will receive new GUID's.

Some may ask why does this matter. If you are using the MIDI controller from Håvard for example, and you are assigning the custom buttons, it would be best to use the GUID instead of Input number so that you have the freedom to change the inputs (ie changing their order on the lower screen) without messing up your custom shortcut buttons. Then by saving the preset you will save the GUID's assigned to each inputs in that particular preset.

As Martin showed, with vMix running you will enter http://localhost:8088/API/ in your browser and that will now show you all of your inputs and GUID's assigned. While this seems so elementary I could not figure out how to find it without asking :)

(example)
<inputs>
<input key="f09bbe41-2230-48f8-9bbd-aa98225031ee" number="1" type="Capture" title="Logitech HD Pro Webcam C920" state="Running" position="0" duration="0" loop="False">Logitech HD Pro Webcam C920</input>
<input key="83ee8c12-f620-4529-83a3-999ab9875b5d" number="2" type="Capture" title="XI100DUSB-HDMI Video" state="Running" position="0" duration="0" loop="False" muted="True" volume="100" solo="False" audiobusses="M">XI100DUSB-HDMI Video</input>
</inputs>

You see the input keys in " " at the beginning.

The more you (I) learn about vMix the more you come to realize what a beast of software this is!! It is fairly easy for the novice and yet so powerful for the advanced. I look forward to continued education and using it to its fullest potential.