As you can see here, you can only specify the update channel via the CitizenFX.ini which is located at your installation path.
BUT - there might be a workaround where you create a small bat script which parses the arguments regarding the update channel, automatically edit the CitizenFX.ini depending on its value and then launching FiveM as normally (running the FiveM.exe) with also the remaining arguments that were passed to the script (things like -cl2 or some +sets).
For changing the update channel via the ini file you might also have to delete the caches.xml file, which was talked about here. But I have no idea if it’s still the case since it’s four years old and some things had changed over time.