The actor/director is fronting the competition, which invites those aged 16-25 to "show up and show off" their talents.
Artists, performers, dancers, musicians and others have been invited to upload a 60-second video to the somewhereto website before July 11.
20 finalists will be chosen from the shortlist and ten of those will go on to perform at a high-profile gig during the Olympic Games in August.
Entrants can also take to the somewhereto_ show off stage, which will visit various venues in Belfast, Edinburgh and Cardiff during June and July.
The overall winner will feature in their own film, directed by Deacon in Rio de Janeiro, Brazil, the host of the 2016 Olympics.
"I'm a massive believer in giving young people the chance to show off what they can do because there's so much undiscovered talent out there," Deacon said.
"I'm really excited to be part of a competition that will be showcasing talented young people throughout the summer."
Artists at last week's launch at St Pancras included soul singer Dionne Reid, champion beatboxer Bass6 and British National Yo-Yo Champion Krisztian Kaluzsa.
CEO of Legacy Trust UK Moira Swinbank said: "Before the Games have even arrived somewhereto_ has already unlocked, for free, over 300,000 square feet of empty or underused space across the UK, the rent value of which would be over £1.6 million.
"As the biggest nationwide youth legacy project we are thrilled to be able to inspire a generation to show off their talent in such a creative and exciting way."
Funded by Legacy Trust UK, somewhereto_ is part of the Cultural Olympiad and London 2012 Festival and an independent charity set up to create a lasting cultural and sporting legacy from London 2012 in communities across the UK.