On Monday, the company announced that it had signed up for a "proposed public listing" with the US Securities and Exchange Commission.
A direct listing can be of use to people who work at Slack because employees and long-term owners can sell shares on the day one. In the case of a traditional stock exchange listing, there is a stock lock period for months before they can sell.
Slack CEO and co-founder Stewart Butterfield is known to say the company is opportunistic to capitalize on people's interest in the service ̵
The company has been vocal about accepting new investment, although it has not yet pressed the vast majority of the money in its warchest.
To date, about $ 1.2 billion has increased from investors, including SoftBank and Accel. In August, the company was valued at $ 7.1 billion.
Slack's messaging service helps employees better collaborate at work. It has been called an "email killer", which gives users a chatroom-like service to cut down on the messy inboxes.
The service has 10 million daily active users globally. Japan is the second largest market with half a million active daily users in the country in July.
Slack relies on businesses to sign up for a paid version of the service to make money. The company said it has 85,000 paid customers worldwide.
Slack, launched in San Francisco in 2013, is one of several technical "unicorns" – the term for private $ 1 billion or more – slated to go public this year. Uber, Lift and Airbnb are also expected to become IPO in 2019.