To use a furniture item on the workshop, you need to get a special item from D.I.Y. There are three workshop items in D.I.Y that support uploaded furniture:
Name
Function
Cost
Workshop Furniture
Allows you to load in a workshop furniture item.
1,200 U
Workshop Physics Item
Allows you to load in a workshop furniture item as a physics item.
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8,000 U
After placing the item down, right/middle click to open its properties menu and select a model from the in-game workshop browser. Keep in mind not to overuse workshop furniture as it can cause significant performance issues when used excessively (100+ unique workshop furniture items in one Condo) compared to official items.