That is a great question and I also use Capture One v21 and I have discouvered how to add the Kelivn slider to Mode 3 of my Quick Keys Dial. Some application are a little more challanging to setup the Dial and Buttons on your Quick Keys. Capture One was not to bad.
With Capture One, the first thing I needed to do was to  create a new Shortcuts menu. If you try to edit the existing one, it will tell you to create a new one to make your edits to first. I called mine Mike 2 .
Then I was able to find the Kelvin setting slider controls under Other.
 I removed the default that was not working for me and added Shift+= which gave me what you see in the box for White Balance -Kelvin +50
I did the same for Kelvin -50 using Shift+minus and what you see in the box is what the application placed there.
You can make other adjustments or at the bottom right, click Save.
We need to set up the Quick Keys to adjust the input levels we set up for Kelvin.
The first step is to add Capture One to the Application row in the Settings program. The link to the right will walk you through that.
I strongly suggest that you add the applications you regularly use to the Application row. It will make your workflow so much better and quicker.
Now that you have Capture One added to the Application row of your Settings program . Click on the icon for Capture One to give it focus ( the yellow bar under the Capture One icon shows it has focus) we can now make a custom change to how the Quick Keys will work with this specific application.
I placed my Kelvin setting adjustment under  Mode 3. You can select any of the four modes to set it up on your Quick Keys.
Once you click on the Mode you want to use. You will see this dialog box appears.
 Shows the Mode you selected. You can think of Mode and the position. As you press the Dial Button, it will move through the four settings and display the Mode name you assigned to it.
 Click on the Selection type, and select Keystroke from the drop-down menu.
 You will want to give it a name that will be easily distinguishable. I named my Kelvin.
 Place your cursor in the top box and then press Shift+=
 Place your cursor in the bottom box and then press Shift+-
 Press OK
And your setting should like the image to the right.
Now, all that is needed is to have Capture One open and make sure you Press the Dial Button until you see the name you used for the feature.
Turn the dial to the Right or the Left to increase or decrease the Kelvin scale within Capture One.
Once you have done this a few times, you will get the hang of where you need to look within an application to located the Shortcut key settings you will need to set up on the Quick Keys.
Please let me now what you think, and if you have any other suggestions.
Im trying to program Quick keys to work with capture one.
To change exposure and such program uses key variables like Add\Substract or OemPlus\OemMinus instead of +\-
Quick keys uses the last one making capture one not to recognize those inputs. any thought how can i register those in quick keys ?