Uploading workshop items is done in-game. Launch Wave Break and navigate to the Park Creator via the main menu.

From here, you can either create a new park or load an existing park. You will find a Park Settings button in the park creator.

https://i.imgur.com/pyIdznN.jpg

In the settings menu, you'll see a "Publish when saving" button.

https://i.imgur.com/akLFu1Q.jpg

When enabled, any time you save the park (as new or overwriting another), Wave Break will also publish the park as a Steam Workshop item.

Once saved and automatically published, Wave Break will automatically subscribe you to your own uploaded workshop item. To view it in-game, you can go to the load park menu, and tab over to the Steam Workshop tab.

https://i.imgur.com/GkbCmfz.png