seting single user mode to beaglebone black in boot prompt

hi gays,

i am trying to set single user mode in beaglebone black but i am not able to set in command prompt at boot time
linux i am new please tell me how to set ,i am using debian 4.9 version also bellow method i am tried but i am not able to set

setenv bootargs single.

hopefully i am waiting for your replay

thanks all