Taken with thanks from this StackOverflow post: http://apple.stackexchange.com/a/241819/132943

Open Terminal.app and enter:

sudo nvram boot-args="-s"

In the future your Mac will always boot to single user mode until you remove (sudo nvram -d boot-args or sudo nvram boot-args="") or replace the boot-args.

Other boot-args directly modifying the boot mode:

sudo nvram boot-args="-v": verbose
sudo nvram boot-args="-x": safe mode