Double them up:
f(SomeJson+"{{0}}");
Or replace them in the JSON, if that's what you need:
f(SomeJson.Replace("{", "{{")
.Replace("}", "}}") + "{0}");
You could also delegate this job to an extension method:
public static class StringExtensions
{
public static string EscapeBraces(this string s)
{
return s.Replace("{", "{{")
.Replace("}", "}}");
}
}
f(SomeJson.EscapeBraces() + "{0}");
Or, as Ergwun says, you could simply concatenate the values afterwards. My assumption, though, is that that's less straightforward in your actual code than in this trivial example.