In CaptureOne, select the eyedropper in the toolbar. This is the Pick White Balance tool. The cursor will become an eyedropper. As you move your cursor over the image, the current values for that area will appear on the window above the image.
For a neutral white balance on the Creative Enhancement target, select either the first patch on the left in the portrait row, or the middle patch on the landscape row. On the ColorChecker Classic target select the second patch in the bottom row.
TIP: White balance can be saved as a preset so you can assign it to other images that were shot in the same lighting. See Automating Color Control in Raw Workflows for details.