How to pass user details from the database?

namespace API
{
    public class DummyPrincipalProvider : IProvidePrincipal
    {
        private const string Username = "###";
        private const string Password = "####";

        public IPrincipal CreatePrincipal(string username, string password)
        {
            if (username != Username || password != Password)
            {
                return null;
                //Page.ClientScript.RegisterStartupScript(GetType(), "done", "alert('your message');", true);
            }

            var identity = new GenericIdentity(Username);

            IPrincipal principal = new GenericPrincipal(identity, new[] { "trial" });
            return principal;
        } 
    }
}

namespace API
{
    using System;
    using System.Collections.Generic;
    
    public partial class api_login
    {
        public string ID { get; set; }
        public string username { get; set; }
        public string password { get; set; }
        public string description { get; set; }
        public string role { get; set; }
    }
}

namespace API
{
    public class BasicAuthMessageHandler : DelegatingHandler
    {
        private const string BasicAuthResponseHeader = "WWW-Authenticate";
        private const string BasicAuthResponseHeaderValue = "Basic";

        public IProvidePrincipal PrincipalProvider { get; set; }

        protected override System.Threading.Tasks.Task<HttpResponseMessage> SendAsync(
            HttpRequestMessage request,
            CancellationToken cancellationToken)
        {
            AuthenticationHeaderValue authValue = request.Headers.Authorization;
            if (authValue != null && !String.IsNullOrWhiteSpace(authValue.Parameter))
            {
                api_login parsedCredentials = ParseAuthorizationHeader(authValue.Parameter);
                if (parsedCredentials != null)
                {
                    Thread.CurrentPrincipal = PrincipalProvider
                        .CreatePrincipal(parsedCredentials.username, parsedCredentials.password);
                }
            }
            return base.SendAsync(request, cancellationToken)
                .ContinueWith(task =>
                                  {
                                      var response = task.Result;
                                      if (response.StatusCode == HttpStatusCode.Unauthorized
                                          && !response.Headers.Contains(BasicAuthResponseHeader))
                                      {
                                          response.Headers.Add(BasicAuthResponseHeader
                                                               , BasicAuthResponseHeaderValue);
                                      }
                                      return response;
                                  });
        }

        private api_login ParseAuthorizationHeader(string authHeader)
        {
            string[] credentials = Encoding.ASCII.GetString(Convert
                                                                .FromBase64String(authHeader))
                .Split(
                    new[] { ':' });
            if (credentials.Length != 2 || string.IsNullOrEmpty(credentials[0])
                || string.IsNullOrEmpty(credentials[1])) 
                return null;
            return new api_login()
                       {
                           username = credentials[0],
                           password = credentials[1],
                       };
        }
    }
}