The spoiler tags thing isn't a button on here (no clue why), but if you write "spoiler" and "/spoiler" (without the quotes) and surround them with [ ] (without the space between them), then it should work.
As to your scripting question, the DisplayMessageBox function takes a String Reference from the dialog.tlk. For experimentation purposes, you might add a new TLK entry to your dialog.tlk using TalkEd, which came with the TSLPatcher.
And no, I don't know if the icon thing works, but knock yourself out.