You don't actually spawn it in, it is it's own .dmx file, meaning that it's a session that you can then manipulate. To get to it, you have to do the following;
When you open SFM, go to "Open Session"
Then, go this filepath " SourceFilmmaker\game\workshop\elements" obviously without the quotation marks
The load up the file "throne.dmx"
And thats it!

Apologies for the late response, I also wondered the same thing as I thought it was a prop, but its an actual save file which is much better in my opinion.
Hopefully this helps you.

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