Creating spawnableable objects for Space is very different from creating objects for more simulation-oriented platforms where such construction activity occurs in-situ. As Unity's method of distinguishing and storing scene elements is the Prefab, as in other application areas, Space picks up the concept and extends it to our use case.
First the object to be used as a Space rezzable is created in Unity. The Space Editor Pack must be imported and and installed; prior to uploading, the creator must be logged in to the Space creator servers.
Once the object's construction has been completed, drag it from the heirarchy back into the project folder where you can find it. Attach the Virtual Good (virtual good) Script component to the prefab of your object.
Examining the Virtual Good (virtual good) Script component in the inspector, fill in the minimum fields: Name, Brand, Developer Name and Category.
Set the Content Type for object type to 'Generic'.
Then click the Automatic Submission button at the bottom of the script's property pane (you might need to scroll the inspector to see it).
This will initiate the item upload, which, you will find, goes a lot more quickly than a region upload. Once it completes, you will find the item in your Space avatar's inventory.
Once you've logged into Space with a web browser or Space desktop client, you will find the object in the avatar's inventory. You can either click it and add it to your hotbar (dragging to the hotbar works too, but be carefull you don't over-write something already there), where it can be clicked for quick spawning, or alternatively, drag the item from your inventory onto the ground.
You can delete the item from your inventory once you have placed it in world.