Reboot directly from MacOS to Startup Manager? (Init command)
A few days ago, I had a problem to reboot on the recovery-mode (CMD+R) and to also to have access to the startup manager (Option/Alt). None of those key combinaisons would work for me. So after some research, I found out how to boot in any drive without the startup manager.  



(Solution 1) Reboot directly onto a USB drive or another partition without the startup manager:
You just need the macOS application Boot Manager. Once installed, it appears in the menu bar, just like the image below. You can  then select the drive that you want to reboot onto.


Here is a tutorial on how to install it: Boot Manager installation tutorial

Note: At some point, you will need to reboot into the recovery-mode. So you can use the following trick 


Open a terminal
Type in: 


  sudo nvram recovery-boot-mode=unused

Enter the admin password
Restart the computer. It will automatically reboot into the recovery-mode




(Solution 2) Replace the startup manager: You can replace the startup manager with an application (just like rEFIND) that will ask you to select the drive to boot onto without having to press the "Option/Alt" key when booting up your computer. The app is Next Loader and is from the same dev. The installation tutorial is here.