This can be done by installing the rEFInd boot manager. This is a free product. In order to bypass the increased security of the newer macOS operating systems, you have to implement a rather complex installation procedure. The steps are outlined below. These steps are just a outline and are not indented to be a detailed installation procedure.
- Download rEFInd.
- Use the Disk Utility,
diskutil command or other method to create a new small (~200 MB) "MS-DOS (FAT)" formatted partition.
- Install rEFInd in to this partition.
- Configure rEFInd to it will appear in the Mac Startup Manager.
- Configure rEFInd to be silent when it boots.
- Restart your Mac and immediately hold down the option key until the Mac Startup Manager icons appear. Select rEFInd as the default operating system to boot to.
- Install the Startup application which can select which operating system rEFInd should silently select for the firmware to boot.
I have found most users will read the above steps and decide to pass on using this method to boot the installed operating systems.
On my Mac, the Startup application appears on the dock as shown below.
When open, the Startup application appears as shown below.
The current version of the Startup application still requires you to manually restart to boot to the selected operating system.
The Startup application is written in Apple Script so you can view the source and easily make any desired modifications.
I suppose the Script Editor preferences could be set to allow opening the Startup application from the menu bar, as shown below.
Or, the Automator application could be used to create a service to open the Startup application. In this case, the Startup application would appear under Services in the menu bar, as shown below.