If you want to use a Bluetooth mouse while you're installing Windows on your Mac, first make sure the mouse is paired in macOS.
When Boot Camp begins Windows installation, the mouse pointer appears on the display. Wait 10 seconds or so, and then left-click the mouse. This wakes the mouse, allowing you to use it to control the pointer.
During installation, only left click and mouse pointer movement work. After Boot Camp successfully installs Windows and the necessary Apple Windows support software, you can use your Bluetooth mouse and all its functions—like right-clicking and scrolling—every time you start up Windows.