Utf8EncodedJson.cs

using System.Text;
using System.Text.Encodings.Web;
using System.Text.Json;
using System.Text.Unicode;

var message = "¡Más amor, por favor!";
var payload = JsonSerializer.Serialize(message, new JsonSerializerOptions
{
    Encoder = JavaScriptEncoder.Create(UnicodeRanges.All)
});
var encodedPayload = Encoding.UTF8.GetBytes(payload);
var encodedMessageText = Encoding.UTF8.GetBytes(message);

Console.WriteLine(ContainsSequence(encodedPayload, encodedMessageText));

return;

bool ContainsSequence(ReadOnlySpan<byte> source, ReadOnlySpan<byte> target)
{
    return source.IndexOf(target) >= 0;
}