CustomAttribute.swift

var string = AttributedString(localized: "This contains my message attribute", including: \.myApp)
let start = string.startIndex
let end = string.characters.index(start, offsetBy: 4)
string[start..<end].myApp.message = Message(id: "message_id", value: "secret")
// print(string.runs):
// This {
//	NSPresentationIntent = [paragraph (id 1)]
//	NSLanguage = en
//	secretMessage = Message(id: "message_id", value: "secret")
// }
// contains my message attribute {
//	NSPresentationIntent = [paragraph (id 1)]
//	NSLanguage = en
// }