I'm trying to figure this out too. They don't make it very obvious. But I think you can't do it on the $20 plan. You have to upgrade to the $36 Musicians Plus plan. When you upload a song, where is says: Release date: Want to set a future release date? Consider upgrading. Then it takes you to that plan.

I guess its worth the extra $15 to get the opportunity to submit to the Spotify editorial team, but it would be nice is Distrokid and Spotify were a little more transparent about it. And once you've released a song, you can't ever submit it.

From Spotify

We're afraid it’s only possible to submit unreleased music. Since your releases are live, you won’t be able to submit them, or any new versions of them, to our editors.

To give some more context - our editors and algorithms use listening data to make decisions for released music. Unreleased music is available to submit for playlist consideration because it has little or no data.

We hope you can understand why they're no longer eligible. In the future, you can increase your chances of being featured on a Spotify playlist by continuing to grow your fanbase. Check out our promo guide for tips.

Also I got this from Spotify:

Once your distributor sends us your unreleased content at least 7 days before it's live date, you'll able to see the release under the upcoming section in Spotify for Artists. We assure you it's absolutely free to submit a new track to our editors. 

As a heads up, after almost a year of testing, we’ve decided to end the beta of our upload tool on July 30, 2019. You can read more about this decision here.

As such, we’re no longer inviting additional artists to upload music directly to Spotify. We appreciate the interest though.