The button to invert the power window is next to the tool that you used to create the selection. You're using the pen tool in this case, but the same is true for ellipses, rectangles, gradients, etc. Immediately to the right of the tool is an icon of a square outline around a solid circle. That's the invert selection button. If you click it before you start your tracking, the tracker will follow everything outside of the shape you drew instead of everything inside the shape.
It's often helpful to toggle "highlight mode" so that you know what's currently selected. Shift+H is the shortcut for that. Everything that's grey with highlight mode on is not part of the current selection. The tracker will follow everything that IS selected, i.e., everything that you see (that isn't grey) when highlight mode is on.
As you become more proficient with power windows, qualifiers, and trackers in Resolve, you'll want to learn how to combine multiple shapes, combine shapes other qualifiers, or combine any/all of the above with alpha channels from external files. The "invert" power window button is a small part of a larger toolset. There's more to explore under the "Key" tab, which will let you control how one node passes a mask or matte to another node. There's also the "Key mixer" node, which allows boolean operations. To access the Key mixer, right-click in the background of the node editor, and select it from the "Add node" submenu.