Boot to Single User Mode with Startup Manager I need enter safe mode on my Mac after selecting select macOS with Startup Manager (option while booting). Pressing ⌘ + S has no effect.

The Mac currently uses Bootcamp for dual boot into Windows or macOS. By default it boots into Windows. I cannot change the default boot OS. Possibly because I don't have an administrator account in macOS. 

The following link is what I'm trying to accomplish using safe mode:

I don&#39;t have administrator account on my mac

TL;DR How to enter macOS safe mode after selecting macOS in Startup Manager?
I am not sure how old your Mac is, but for Macs made in at least the last 10 years, this should work.

After booting to the Startup Manager, you can hold down the control key while selecting the operating system to boot. You should see the straight arrow change to a circular arrow. The selected operating system will now become the default.

Additionally, many Boot Camp arrangements allow the default boot operating system to be selected from the Windows Control Panel.