You are not looking for a constant; a handle is typically acquired at runtime. In this case, by calling other functions of the Windows API.
Unfortunately, the documentation you need is not available on docs.rs. windows and windows-sys crates are generated from the Windows API metadata, and do not include any rustdoc. Instead, you need to go on https://learn.microsoft.com.
The documention for SetMonitorBrightness is found here.
You’ll find information on how you can retrieve a handle properly:
I suggest you dig the documentation from here. At some point you will enumerate the monitors, filter out what does not interest you (you may receive pseudo-monitors), and so on until you have everything you need to call SetMonitorBrightness, i.e., the handle for the monitor you are interested in.
You may also want to call GetMonitorBrightness in order to find what are the minimum and maximum brightness values.