Do you mean how do you extract the kid
from the JWT header? It's just a base64 url-encoded JSON string, so you can just decode & deserialise it:
using Microsoft.IdentityModel.Tokens;
using System.Text.Json;
const string jwt = "eyJraWQiOiIxZTlnZGs3IiwiYWxnIjoiUlMyNTYifQ.ewogImlzcyI6ICJodHRwOi8vc2VydmVyLmV4YW1wbGUuY29tIiwKICJzdWIiOiAiMjQ4Mjg5NzYxMDAxIiwKICJhdWQiOiAiczZCaGRSa3F0MyIsCiAibm9uY2UiOiAibi0wUzZfV3pBMk1qIiwKICJleHAiOiAxMzExMjgxOTcwLAogImlhdCI6IDEzMTEyODA5NzAsCiAiY19oYXNoIjogIkxEa3RLZG9RYWszUGswY25YeENsdEEiCn0.XW6uhdrkBgcGx6zVIrCiROpWURs-4goO1sKA4m9jhJIImiGg5muPUcNegx6sSv43c5DSn37sxCRrDZZm4ZPBKKgtYASMcE20SDgvYJdJS0cyuFw7Ijp_7WnIjcrl6B5cmoM6ylCvsLMwkoQAxVublMwH10oAxjzD6NEFsu9nipkszWhsPePf_rM4eMpkmCbTzume-fzZIi5VjdWGGEmzTg32h3jiex-r5WTHbj-u5HL7u_KP3rmbdYNzlzd1xWRYTUs4E8nOTgzAUwvwXkIQhOh5TPcSMBYy6X3E7-_gr9Ue6n4ND7hTFhtjYs3cjNKIA08qm5cpVYFMFMG6PkhzLQ";
string[] sections = jwt.Split(new[] { '.' });
IDictionary<string, object> header = JsonSerializer.Deserialize<IDictionary<string, object>>(Base64UrlEncoder.Decode(sections[0]));
IDictionary<string, object> payload = JsonSerializer.Deserialize<IDictionary<string, object>>(Base64UrlEncoder.Decode(sections[1]));
string algorithm = header["alg"].ToString();
string kid = header["kid"].ToString();
string issuer = payload["iss"].ToString();
Console.WriteLine(kid);
Or do you mean where does kid
come from in the first place?