Creating a body from a string can be accomplished using BodyPublishers.ofString(), as shown in the following snippet of code:

HttpRequest requestBody = HttpRequest.newBuilder()
  .header("Content-Type", "application/json")
  .POST(HttpRequest.BodyPublishers.ofString(
    "{"name": "morpheus","job": "leader"}"))
  .uri(URI.create("https://reqres.in/api/users"))
  .build();

For specifying a charset call, use ofString(String s, Charset charset).