Add a new class called SwaggerBasicAuth and add the following code.

    public class SwaggerBasicAuth
    {
        private readonly RequestDelegate next;
        public SwaggerBasicAuth(RequestDelegate next)
        {
            this.next = next;
        }
        public async Task InvokeAsync(HttpContext context)
        {
            if (context.Request.Path.StartsWithSegments("/swagger"))
            {
                string authHeader = context.Request.Headers["Authorization"];
                if (authHeader != null && authHeader.StartsWith("Basic "))
                {
                    // Get the credentials from request header
                    var header = AuthenticationHeaderValue.Parse(authHeader);
                    var inBytes = Convert.FromBase64String(header.Parameter);
                    var credentials = Encoding.UTF8.GetString(inBytes).Split(':');
                    var username = credentials[0];
                    var password = credentials[1];
                    // validate credentials
                    if (username.Equals("Swagger") && password.Equals("Shubham123"))
                    {
                        await next.Invoke(context).ConfigureAwait(false);
                        return;
                    }
                }
                context.Response.Headers["WWW-Authenticate"] = "Basic";
                context.Response.StatusCode = (int)HttpStatusCode.Unauthorized;
            }
            else
            {
                await next.Invoke(context).ConfigureAwait(false);
            }
        }
    }

    public static class SwaggerExtensions
    {
        public static IApplicationBuilder UseSwaggerAuthorized(this IApplicationBuilder builder)
        {
            // Your middleware registration goes here
            return builder.UseMiddleware<SwaggerBasicAuth>();
        }
    }

Open the program.cs file and add the following code 

builder.Services.AddSwaggerGen(c =>
{
    c.SwaggerDoc("v1", new Microsoft.OpenApi.Models.OpenApiInfo { Title = "SwaggerAuth", Version = "v1" });
});

    app.UseSwaggerAuthorized();
    app.UseSwagger();
    app.UseSwaggerUI(c =>
    {
        c.SwaggerEndpoint("/swagger/v1/swagger.json", "SwaggerAuth V1");
    });

Complete code of the program.cs

using SwaggerAuth;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddControllers();
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();

builder.Services.AddSwaggerGen(c =>
{
    c.SwaggerDoc("v1", new Microsoft.OpenApi.Models.OpenApiInfo { Title = "SwaggerAuth", Version = "v1" });
});


var app = builder.Build();

// Configure the HTTP request pipeline.

 app.UseSwaggerAuthorized();
app.UseSwagger();
app.UseSwaggerUI(c =>
  {
        c.SwaggerEndpoint("/swagger/v1/swagger.json", "SwaggerAuth V1");
  });


app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();