It's in the crosspost settings. They have a default footer you can use, and they also supply bits of code for the variable things, to be entered in a helpful box.
OK, I see a helpful box for appending a footer to the other post, but nothing for the local copy saying "Crossposts: blah.livejournal.blah/12345.html". Where dat?
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HELPFUL BOX IS HELPFUL.
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...oh that helpful box
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