But Lachlan Robert Wilson, 18, continues to deny an allegation that he also blackmailed the 61-year-old victim.

Wilson pleaded guilty in the ACT Magistrates Court on Tuesday to a charge of aggravated robbery, admitting he stole $100 while threatening to use force and in company with a co-offender.

Magistrate James Stewart committed him to the ACT Supreme Court for sentence on that charge.

Mr Stewart also committed Wilson for trial on a blackmail charge, after defence lawyer Daniel Turner said the teenager would maintain his previously entered plea of not guilty on that count.

The precise facts of Wilson's admission are yet to be determined, but police alleged in the aftermath of his November arrest that he targeted the victim through a site called "squirt.org".

In documents tendered to the court, police claimed Wilson exchanged messages with the victim using the name "youngboihorny".

He allegedly told the victim he was 15 years old and wanted to meet, and police said the victim only agreed because he believed he was being "set up" and wanted to expose whoever was posing as a child.

According to police, Wilson eventually used a phone number registered in his name to call the victim and set up a meeting at a unit complex in the Belconnen suburb of Hawker.

Investigators claimed that when the victim arrived at the agreed location on November 5, he was "confronted by two males".

It was alleged that Wilson and a 16-year-old boy, who cannot be named for legal reasons, threatened the victim with a steak knife and called him a paedophile before leading him to an ATM at the Hawker shops and demanding money.

Police said in court documents that the victim withdrew $100 and gave it to his assailants, leaving only $17 in his account.

The pair then allegedly told the victim they would be in touch to collect more money from him in the future, threatening to "expose [him] as a paedophile if he did not pay".

Investigators said the victim quickly drove to Belconnen Police Station and made a statement, while also allowing police to take control of his mobile phone.

An Australian Federal Police covert operative used the device, according to court documents, to text the number registered to Wilson and ask: "How can I make this go away?"

Wilson allegedly said in a subsequent series of messages that it would not be going away "for a long time" unless he got "a fare [sic] share of money".

He is said to have suggested ongoing payments or "one big sum to be left alone for good".

Police arrested Wilson at a Narrabundah hotel a few days after the robbery.

Now on bail, he is due to appear in the Supreme Court for an administrative hearing next Thursday.

The 16-year-old boy has also been charged, and is before the ACT Children's Court.

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