It comes ahead of November's Budget, with the chancellor under pressure to balance the public finances, while also boosting economic growth.
Reeves said no companies had signed up to the scheme yet as it has not been formally announced, but added that several business organisations had come out in support.
The initiative builds on a "youth guarantee", announced last November, which promised every 18 to 21-year-old in England access to an apprenticeship, training, education opportunities or help to find a job.
Under the new plans, every young person who has been on Universal Credit for 18 months without "earning or learning" will be offered a guaranteed paid work placement.
Those who refuse to take up the offer without a reasonable excuse will face sanctions such as losing their benefits.
The aim of the placements would be to help people build up the skills to get a full-time job.
An estimated one-in-eight 16 to 24-year-olds are not currently in education, employment or training - around 948,000 people - according to the latest figures.
The numbers hit an 11-year high of 987,000 at the end of last year.
