By Joshua Franklin

NEW YORK (Reuters) - Slack Technologies, the owner of the workplace instant messaging app, plans to go public on June 20, the company said on Monday, a test of investor appetite for loss-making technology stocks in the wake of Uber Technologies Inc's underwhelming market debut.

Slack is one of the most high-profile companies left to go public in 2019 and its debut could be a bellwether for other tech listings this year.

So far in 2019, companies such as Pinterest Inc, Zoom Video Communications and Beyond Meat have gone public and seen their stocks trade up.

However, the struggles of Uber and smaller ride-hailing rival Lyft Inc, the two biggest initial public offerings (IPOs) so far in 2019, have cast a pall over new tech listings.

Uber shares fell as much as 10% on Monday, more than doubling their losses since the ride-hailing giant's poorly received Wall Street debut last week and raising more questions about investors' faith in its ability to make profits.

San Francisco-based Slack reported losses from operations of $143.85 million for 2018, a fraction of the $3 billion lost from operations by Uber in 2018.

Slack is seeking to go public via a direct listing instead of an IPO, which has been the traditional route to the public markets for companies like Google parent Alphabet, Facebook Inc and Uber.

Unlike a traditional IPO in which companies sell shares to raise proceeds, a direct listing is purely a way for existing shareholders to sell stock. Music streaming app Spotify Technology SA last year went public via a direct listing.

The June listing date could still be subject to change, Jesse Hulsing, Slack vice president of investor relations, told an investor event, which was webcast.

In an updated regulatory filing on Monday, Slack said first-quarter revenue grew by around 65% year-on-year to around $134 million. Losses from operations also widened by almost 50% to roughly $39 million.

Slack said it plans to report detailed first-quarter results on June 10.

It plans to trade on the New York Stock Exchange under the symbol "SK".

The company has hired Goldman Sachs, Morgan Stanley, and Allen & Company to be financial advisors on the direct listing, which requires a fraction of the number of banks as an IPO.

(Reporting by Joshua Franklin in New York; Editing by Nick Zieminski)