Get the ID of the object by going to
https://powdertoy.co.uk/User/Saves.html?ID=5705
Then click on the Save you want to embed (like https://powdertoy.co.uk/Browse/View.html?ID=228602) and get the ID
(The ID of this object is 228602).

Then type ~ (ID)

So something like ~ 228602 (Without the space)
turns out to be