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You have to use the run-script x command within the dialog. There's a dialog request-section in the main script where you can put every code you need.
Do it like this:
In dialog 1 put: run-script 1
In the dialog-request section of the main script put:
if (script == 1) {
dialog[2].SetOptionState(4, eOptionOn);
}
..which will turn on option 4 in dialog 2.
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Change "parameter" to "script", then it should work.
But if you post the run-script command in the dialog, the dialog_request-section should appear automatically.
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I'd prefer a drink.. ;)