"Swallow it," they can be heard telling the boy on the video that was later uploaded to social media as they laugh.
It was among a number of degrading acts the 17-year-old was forced to perform after being threatened with a knife on a Brisbane train.
Footage uploaded to Snapchat was shown in Brisbane District Court of the teenager eating the "dumper" cigarette butt, taking off his pants and then kissing a shoe worn by one of the offenders.
He was also forced to send a message to his girlfriend, breaking up with her.
Knox and the co-accused approached the 17-year-old who was sitting alone on a Brisbane train in March 2022, the court heard.
As the train approached a station in Brisbane's south, the boy was told: "You are getting off the train and you are coming with us".
The co-offender then lifted his shirt to reveal a knife tucked into his pants.
Knox took the boy's bag which contained a PlayStation 4 game console, two controllers, a wallet and phone charger.
Crown prosecutor Carla Ahern said when they got off the train they took his phone and subjected the boy to the humiliating acts as they filmed, calling him "fat boy" and "c***".
"This is going on your Snapchat bro," the boy is told on one of the videos that was later uploaded by the offenders.
They told the 17-year-old they would go to his house and "f*** you up and bash your dad and rape your f***ing sister" if he told anyone what happened.
The two co-offenders walked back to the station with the boy and later pushed him off a train at a stop after also taking his shoes.
The boy immediately told railway staff and Knox and the co-offender were arrested and detained by police at a stop heading to the Gold Coast.
"The offending is disgusting ... the conduct is appalling," defence barrister David Funch said.
"When you look at this sort of offending it is very easy ... to get carried away by the depravity of it all and not exercise appropriate restraint."
Mr Funch said Knox was homeless from 13 to 16 and had been addicted to methamphetamine.
Knox, 19, pleaded guilty to armed robbery in company, deprivation of liberty and stealing.
Judge Michael Burnett sentenced him to three years in jail, suspended for four years after serving 364 days.
He also received a three-year probation term under strict conditions including drug testing.
Knox's 364 days spent in custody was declared time served.
He remains in custody for other matters.
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