Go to Settings / Customize Toolbars... in your DOpus menus. Right-click on a toolbar and select either New / New Button (if you right-clicked on an empty area of a toolbar) or Insert New / New Button (if you right-clicked directly on an existing button, this will insert the new button to the right of the button you right-clicked on).
You'll now have only a lovely button named "New Button". If we stopped here, the New Button would do nothing.
With the Customize window still open, right-click your "New Button" and choose Edit. This will give you a blank Command Editor for that button.
The Label is the "New Button" text we see on the button, change that to something appropriate like "Go C:". I filled in the Tip with useful text that will show up when you hover over the button. Then I coped Leo's command above and pasted it into the Function box. (I also picked an icon. You can click the left icon with the 1 in the corner and pick something. I searched for "drive" in the icon list and picked one of those.)
Here's my handy-dancy new button...
Don't forget to close the Customize window.
Now, when clicking the "Go C:" button, I get a new DOpus window and it's already anchored to the C:\ location.
(Tip: If it's in your way while editing the toolbars, you can move the Customize window.)