Soundiiz is supporting the export of Serato playlists to streaming services with the URL link provided by Serato on his website Serator.com.
For that, go to your playlist on Serato, be sure your playlist is "public" and copy the URL (which looks like https://serato.com/playlists/userID/mixDate).
Then simply go back to Soundiiz, tab Playlists and use the "Import playlist" button in the top right of the interface and select "By URL". Then paste your URL and follow the steps!