Double quotes in c# doesn't allow multiline

If you really need to do this in a string literal, I'd use a verbatim string literal (the @ prefix). In verbatim string literals you need to use "" to represent a double quote. I'd suggest using interpolated string literals too, to make the embedding of title and message cleaner. That does mean you need to double the {{ and }} though. So you'd have:

string title = "This is the title: ";
string message = "(Message)";
string str = $@"
{{
   ""aps"":
   {{
       ""alert"":""{title}{message}""
   }}
}}";
Console.WriteLine(str);

Output:

{
   "aps":
   {
       "alert":"This is the title: (Message)"
   }
}

However, this is still more fragile than simply building up JSON using a JSON API - if the title or message contain quotes for example, you'll end up with invalid JSON. I'd just use Json.NET, for example:

string title = "This is the title: ";
string message = "(Message)";
JObject json = new JObject
{
    ["aps"] = new JObject 
    { 
        ["alert"] = title + message 
    }
};
Console.WriteLine(json.ToString());

That's much cleaner IMO, as well as being more robust.