test.cs

[HttpGet]
public IActionResult ReplaceWord(string word) {

  Dictionary<string, string> dict = new Dictionary<string, string>()
  {
     { "Word1", "<strong>Formatted</strong> replacement for <em>Word1</em>" },
     { "Word2", "<strong>Formatted</strong> replacement for <em>Word2</em>" },
     { "Word3", "<strong>Formatted</strong> replacement for <em>Word3</em>" }
  };

  return dict.ContainsKey(word) ? Ok(dict[word]) : Ok(null);
}