From today until 18 June, commuters and passers-by will be invited to step inside the digital installation, take a seat and answer the question: "If you had one thing that you could tell the next generation, what would it be?" via a 15-second video.
The capsule can hold up to three people at a time, enabling friends and family members to leave a joint message.
Visitors have complete creative freedom in terms of the content of their messages. They can be about a particular place or places, technology, music, culture, or an insight for the future.
The messages will revealed in 50 years time - the year 2065 - and read by future generations.
During the media launch brand, ambassador John Leith explained Hennessy created a special cocktail, coined the Hennessy Crusta, to celebrate its 250th anniversary.
"It is based on the brandy Crusta - one of the first ever cocktails," he said. "It is a celebratory drink for this year."
Those who leave a message will be able to sample the one-off cocktail from the St Pancras Rennaisance Hotel, which is situated above the time capsule.
The time barrel has arrived from Russia, and Hennessy representatives worked through the night (11 June) to piece the installation together over approximately eight hours.
Hennessy opted for St Pancras not only because it acts as a gateway to Cognac, France – the brand’s homeland – but as it is a destination among travellers and a melting pot of different cultures.
The time barrel departed Cognac, France in February to embark on a global tour in celebration of Hennessy's 250th anniversary.
It has since travelled to China, Taiwan, Berlin, Germany and Russia. Following its stint in London the time barrel will stop off in Canada, South Africa and finally Lagos in October.